Multipass graphics rendering method and apparatus with re-traverse flag

ABSTRACT

A graphics rendering system allows an application program to make immediate or retained mode calls to render a model, without needing to know how many passes the renderer requires to complete the scene. The application program invokes the rendering subsystem and the rendering subsystem returns a re-traverse flag indicating whether the rendering of the model is complete. If the flag indicates that rendering is not yet complete, the application program again invokes the rendering subsystem. Calls to the rendering subsystem may be placed inside a loop in the application program, which repeats until the re-traverse flag indicates completion. When the re-traverse flag indicates that rendering is not yet complete, the application program repeats the same sequence of calls, thereby effectively re-traversing the model. Application program calls to the rendering system can also specify the renderer to use. In this manner, switching to a different renderer at any time during the building or editing of a model becomes a trivial task for the application program. Also, more than one renderer can be active simultaneously. The current state of rendering for each renderer is stored in a respective &#34;view object&#34; (or objects) which the application program specifies when calling the rendering system. The system is also extensible to support dynamically registered renderers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application SC/Ser. No. 08/362,118, filed Dec. 22, 1994, entitled MECHANISM FOR IMPLEMENTING SCALABLE AND EXTENSIBLE GRAPHICS RENDERING, by inventor David Jevans, assigned to the assignee of the present application and incorporated herein by reference.

LIMITED COPYRIGHT WAIVER

A portion of the disclosure of this patent document contains material to which the claim of copyright protection is made. The copyright owner has no objection to the facsimile reproduction by any person of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office file or records, but reserves all other rights whatsoever.

BACKGROUND

1. Field of the Invention

The invention relates to graphics rendering systems, and more particularly, to software tools for assisting graphics application developers.

2. Description of Related Art

Graphics rendering is the process of computing a two-dimensional image (or part of an image) from three-dimensional geometric forms. An object is considered herein to be three-dimensional if its points are specified with at least three coordinates each (whether or not the object has any thickness in all three dimensions). A renderer is a tool which performs graphics rendering operations in response to calls thereto. Some renderers are exclusively software, some are exclusively hardware, and some are implemented using a combination of both (e.g. software with hardware assist or acceleration). Renderers typically render scenes into a buffer which is subsequently output to the graphical output device, but it is possible for some renderers to write their two-dimensional output directly to the output device. A graphics rendering system (or subsystem), as used herein, refers to all of the levels of processing between an application program and a graphical output device. In many prior art systems, the graphics rendering system is coextensive with the renderer; that is, the renderer is called directly by the application program without any intervening layers of processing.

Graphics rendering systems typically feature an immediate mode interface or a retained mode interface to the application program. An immediate mode interface is a truly procedural interface in which the application program specifies each geometric primitive to the graphics rendering system every time the image is to be rendered. The rendering system does not maintain a model database from scene-to-scene, although the application program may do so. Immediate mode interfaces are highly attractive for rendering scenes where the model is changing at each frame, such as for visualization of simulations, previewing of animation sequences, or reading a series of models from a file. On the other hand, an immediate mode interface requires that the entire scene be transmitted via procedure calls to the renderer at each frame, resulting in high data bandwidth between application program and renderer. Also, the file format for a model is often simply a stream of drawing commands rather than the model itself, restricting its usefulness as a data interchange format. Immediate mode interfaces are also less conducive to providing toolkit modeling functionality to the application program, and they usually preclude a user interface toolkit that operates on objects in the scene.

In a retained mode system, sometimes called a display list system, the graphics rendering system maintains a database representation of the three dimensional model. At each frame, the rendering system traverses the retained model database and draws it. This can be instigated by a single call by the application program to the graphics rendering system, instead of a stream of drawing calls describing the entire scene. When the model changes, the application program edits or updates the model database and again asks the rendering system to render the scene. The benefits of a retained mode system include reduced bandwidth between the application program and any hardware accelerator. The file format of the model database also can be used easily as a data interchange format since it is not merely a list of procedure calls. The existence of an object database also provides an additional way of implementing a user interface toolkit and modeling functionality. Retained mode renderers can also cache rendering information and can also cache information for optimization of scene traversal. On the other hand, retained mode rendering systems have a higher overhead for editing the scene database, and they restrict application program design by forcing the scene into a system-defined data structure, usually a hierarchy, thus requiring many application programs to maintain a duplicate copy of the model in their own format.

In Mark A. Tarlton and P. Nong Tarlton, "A Framework for Dynamic Visual Applications," Proceedings of the 1992 Symposium on Interactive 3D Graphics, Cambridge, Mass., 1992, pp. 161-164, incorporated by reference herein, there is described a retained mode rendering system which implements a general purpose database system to organize the model rather than forcing the model to reside in a single system hierarchy. Such a technique attempts to provide the benefits of the retained mode system without the drawbacks. Such a high level scheme may not solve the model organization problem for all applications, however, and is not optimum for visualization applications where the scene is changing at each frame.

Conventional graphics rendering systems have a number of additional problems which limit their usefulness in one way or another. The following list describes some of the graphics rendering systems which are currently available.

GL™. Silicon Graphics' GL is an immediate mode renderer used primarily for interactive graphics. GL is described in "Graphics Library Programming Guide," Silicon Graphics Computer Systems, 1991, incorporated by reference herein. It was designed as an interface to Silicon Graphics IRIS rendering hardware and does not provide a file format, hard copy output, modeling capability, or user interface tools. As its design stems from a hardware implementation, a software implementation of GL is suboptimal since caching and aggregate primitives such as polygon meshes are not supported. GL supports simple display lists which are essentially macros for a sequence of GL commands. The GL routines perform rendering operations by issuing commands to the IRIS hardware.

StarBase™. Hewlett-Packard's StarBase is an immediate mode system that is very similar to GL, sharing most of its features and disadvantages. StarBase is described in "Starbase Graphics Techniques, Hewlett-Packard Company," 1991, incorporated by reference herein. Numerous device drivers are available for StarBase for outputting the rendered (i.e. two-dimensional) scene on different graphics output devices ranging from plotters to high-end 3-D graphics work stations.

RenderMan™. RenderMan, by Pixar, is an immediate mode system designed primarily to support high quality rendering. RenderMan is described in Steve Upstill, "The RenderMan Companion," Addison-Wesley, Reading, Mass., 1990, incorporated by reference herein. While RenderMan does provide a simple mechanism for making display lists, no manipulation of a RenderMan display list is supported. RenderMan does not provide any support for interaction such as picking (object selection from a mouse click). As described in Tony Apodaca, "RenderMan Interface Specification Version 4.0 Beta," January, 1992, incorporated by reference herein, recent versions of the RenderMan specification provide new routines that bracket the existing RenderMan calls and allow different renderers to be used. The renderer is specified with a single call prior to rendering the scene, and it affects the entire scene. See also Pixar, "Quick RenderMan Interface and Implementation Specifications" 1992, incorporated herein by reference.

PHIGS. PHIGS is described in PHIGS Committee, A. van Dam, chair, "PHIGS Functional Description, Revision 3.0," Computer Graphics, 22(3), 1988, pp. 125-218, incorporated by reference herein, and is a descendent of GKS-3D, described in International Standards Organization, "International Standard Information Processing Systems Computer Graphics - Graphical Kernel System for Three Dimensions (GKS-3D) Functional Description," ISO Document Number 8805:1988(E), American National Standards Instituted New York, 1988, incorporated by reference herein. PHIGS was a committee-designed system for interactive 3-D graphics display. In PHIGS, the entire model database resides in a single hierarchy. Application programmers must learn a host of editing and hierarchy manipulation calls in order to effectively use the system. PHIGS employs a single renderer that supports all the rendering modes specified available in PHIGS, and does not support alternative renderers for photorealism or other effects.

PEX. PEX is an extension to the X-Windows system, defined by a serial protocol (for transmitting data between an application program and the X-Windows system) and a set of semantics which were originally derived from PHIGS. PEX has several available APIs, all of which support retained-mode, immediate-mode, and mixed-mode function calls for drawing, changing state, etc. PEX is described in "PEX Protocol Specification, Version 5.0P-X Public Review Draft," 14 Sep., 1990, Massachusetts Institute of Technology, incorporated by reference herein.

HOOPS™. HOOPS, by Ithaca Software, is described in Garry Wiegand and Bob Covey, "HOOPS Reference Manual, Version 3.0," Ithaca Software, 1991, incorporated by reference herein. It is a retained mode 3-D graphics system, which organizes the model in a hierarchy whose nodes are accessed through textual strings in much the same way that files in the UNIX file system are referenced. Like PHIGS, HOOPS supports a single renderer. However, HOOPS provides more extensive scene editing functionality than PHIGS.

DORE™. DORE, by Kubota, is an example of a 3-D graphics system with an object-oriented design. It is described in "Dore Programmer's Guide," Release 5.0, Kubota Pacific Computer Inc., 1991, incorporated by reference herein. DORE was designed so that scene data is renderable by many kinds of renderers, rather than a single monolithic renderer as provided by PHIGS. Renderers cannot be added dynamically to DORE, however, as the rendering methods are built into the system. In DORE, the choice of renderers is specified by setting the current rendering style in the DORE "view object". DORE then also requires the application program to attach the model to the view object before rendering. This restricts DORE to utilizing only one renderer at a time. There are other design considerations in DORE that also restrict it to using only one renderer at a time; for example, only one set of global variables is provided for maintaining the rendering state.

DORE is a retained mode system. To relieve much of the hassle associated with editing the model hierarchy and to facilitate dynamic databases and user interaction, DORE supports application callback objects, whereby an application program defines a function to be called when the callback object is encountered during scene traversal.

Inventor™. Inventor is an object oriented 3-D graphics user interaction toolkit that sits on top of the GL graphics system. Like DORE, Inventor supports multiple renderers by having a renderer-specific "render" method for each object type. Inventor is a retained mode system with the entire scene residing in a "scene graph". Inventor has render action objects that take a model as a parameter. The renderer is selected by the rendering action that is used when drawing the model. The render action draws the entire model by traversing the model and calling the appropriate rendering method for each object. The usual render action is the GL rendering mode. Inventor is further complicated by including user interface widgets as objects imbedded in the scene graph. As such, implementing highly dynamic applications such as animation, scientific visualization or implicit surface modeling can be difficult. Inventor is described in Wernecke, "The Inventor Mentor", Addison-Wesley (1994), incorporated by reference herein.

Other references pertinent to the disclosure herein are the following, all incorporated by reference herein: Bergman, Fuchs, and Grant, "Image Rendering by Adaptive Refinement," Computer Graphics, 20(4), 1986, pp. 29-37; Catmull, "A Subdivision Algorithm for Computer Display of Curved Surfaces," Ph.D. Thesis, Report UTEC-CSc-74-133, Computer Science Department, University of Utah, Salt Lake City, Utah, December, 1974; Chen, Rushmeier, Miller, and Turner, "A Progressive Multi-Pass Method for Global Illumination," Computer Graphics, 25(4), 1991, pp. 165-174; Clark, "The Geometry Engine: A VLSI Geometry System for Graphics," Computer Graphics, 16(3), 1982, pp. 127-133; Foley, van Dam, Feiner, and Hughes, "Computer Graphics: Principles and Practice," Addison-Wesley, Reading, Mass., 1990; Haeberli and Akeley, "The Accumulation Buffer: Hardware Support for High-Quality Rendering," Computer Graphics, 24(4), 1990, pp. 309-318; Kelley, Winner, and Gould, "A Scalable Hardware Render Accelerator using a Modified Scanline Algorithm," Computer Graphics, 26(2), 1992, pp. 241-248; Maillot, "Three-Dimensional Homogeneous Clipping of Triangle Strips," Graphics Gems II, Academic Press, Inc., San Diego, Calif., 1991, pp. 219-231; Newell, Newell, and Sancha, "A Solution to the Hidden Surface Problem," Proceedings of the ACM National Conference, 1972, pp. 443-450; Potmesil and Hoffert, "FRAMES: Software Tools for Modeling, Rendering, and Animation of 3D Scenes," Computer Graphics, 21(4), 1987, pp. 85-93; Saito and Takahashi, "Comprehensible Rendering of 3-D Shapes," Computer Graphics, 24(4), 1990, pp. 197-206; Segal, Korobkin, van Widenfelt, Foran, and Haeberli, "Fast Shadows and Lighting Effects Using Texture Mappings," Computer Graphics, 26(2), 1992, pp. 249-252; Sillion and Puech, "A General Two-Pass Method Integrating Specular and Diffuse Reflection," Computer Graphics, 23(3), 1989, pp. 335-344; Snibbe, Herndon, Robbins, Conner, and van Dam, "Using Deformations to Explore 3D Widget Design," Computer Graphics, 26(2), 1992, pp. 351-352; Strauss and Carey, "An Object-Oriented 3D Graphics Toolkit," Computer Graphics, 26(2), 1992, pp. 341-349; Turkowski, "Design Considerations for an Object-Oriented [3D Graphics] Metafile," Proceedings of the Third Eurographics Workshop on Object-Oriented Graphics, Charnpery, Switzerland, October, 1992, pp. 163-169; Venolia, "Facile 3D Direct Manipulation," to appear in Proceedings of CHI '93, ACM/SIGCHI, Amsterdam, May, 1993; Venolia, "Automatic Alignment in Two and Three Dimensions," submitted to SIGGRAPH '93; and Wanger, "The Effect of Shadow Quality on the Perception of Spatial Relationships in Computer Generate Imagery," Proceedings of the 1992 Symposium on Interactive 3D Graphics, Cambridge, Mass., 1992, pp. 3942.

It is desirable to be able to draw a model using a variety of different kinds of renderers. For example, an application program which permits interactive editing of a three-dimensional model may benefit by using a high-speed, low-quality wire frame renderer to draw intermediate versions on a display, and by using a high-quality, low-speed Z-buffer renderer to draw the final version on a printer. However, the interchangeability of renderers is awkward at best using the rendering systems described above. Specifically, some of the above rendering systems do not support more than one renderer, while those which do, such as DORE and Inventor, bind the chosen renderer into the system configuration. Accordingly, there is a need for a graphics rendering system which provides increased flexibility to use different renderers for different purposes on the same three-dimensional model.

Another problem with the above rendering systems is that they do not support more than one renderer being active simultaneously. Simultaneous rendering of a model is desirable in a number of situations including simultaneous output of an image on both a display and a printer. As another example, it is sometimes desirable to render two different views of a model at the same time on two different parts of the same display. Simultaneous rendering would be desirable primarily in immediate mode systems, in which an application program draws an image by making a sequence of calls to the rendering system. In such systems, many application programs would be able to execute more efficiently by interspersing calls for one renderer with the sequence of calls for another renderer. Prior systems precluded such interspersed calling sequences.

Yet another problem with the above rendering systems is that those systems which supported more than one renderer, required all renderers to support at least the same geometries. For example, the Inventor rendering system could support only renderers which were able to render points, lines, and certain other predefined shapes. Simpler renderers could not be used with Inventor. More capable renderers could be used, as long as they support at least the full set of Inventor's geometric primitives, but they could not be added dynamically. The application program would have to be aware of these renderers a priori. The above rendering systems did not permit "plug-in" rendering of more or less capable renderers, with automatic detection and utilization of the renderer's features.

Yet another problem with the above rendering systems is that they do not support multipass rendering well, especially not those which support immediate- and mixed-mode rendering. A multipass renderer is one which is designed to require multiple traversals of a scene database. There are a number of reasons why a renderer might be designed for multiple passes on the scene database. For examples there may not be enough memory to render the entire frame buffer if rendering at very high resolutions. A renderer might then separate the frame buffer into strips and render each strip via a separate pass through the scene database. As another example, if the image is being rendered to multiple devices, each at a different resolution (such as when a window overlaps two monitors or when an image is being rendered to both the screen and a high resolution printer), the renderer may render to each device in a separate pass. As another example, a renderer that implements progressive refinement may make multiple passes on the scene database, continually refining the final image until the process is stopped by a user action or the image is of highest quality. Still other examples include high quality rendering algorithms involving accumulation buffers, involving using texture mapping to create shadowing and lighting effects, and involving two-pass global illumination techniques.

In a retained mode system, the application program usually does not need to know whether the renderer requires more than one pass to complete its rendering. This is because the application program makes only one call to cause the renderer to render the model, and the renderer can make as many passes as it requires before returning to the application program. In an immediate mode or mixed mode system, on the other hand, the application program plays a part in traversing the model, making multiple calls to the renderer to accomplish a single traversal. Thus for a multipass renderer, the application program would need to know how many times to traverse the model in order to complete the scene. This would require the application program to have an intimate knowledge of the renderer, and essentially would make it very difficult to support interchangeable renderers.

SUMMARY OF THE INVENTION

According to the invention, roughly described, a graphics rendering system is provided which allows an application program to make immediate or retained mode calls to render a model, without needing to know how many passes the renderer requires to complete the scene. In one embodiment, the application program invokes the rendering subsystem to draw the model, and the rendering subsystem returns a flag (referred to herein as a re-traverse flag) indicating whether the rendering of the model is complete. If the flag indicates that rendering is not yet complete, the application program can again invoke the rendering subsystem to draw the model. Preferably, the calls to the rendering subsystem are placed inside a loop in the application program, which repeats until the re-traverse flag indicates completion. The technique supports immediate and mixed mode rendering because the loop can contain many calls to the rendering subsystem, all made by the application program in traversing the model itself. When the re-traverse flag indicates that rendering is not yet complete, the application program repeats the same sequence of calls, thereby effectively re-traversing the model.

In an aspect of the invention, application program calls to the rendering system to draw an object specify not only the object to draw, but also the renderer to use to do so. In this manner, switching to a different renderer at any time during the building or editing of a model becomes a trivial task for the application program. Because of the re-traverse flag, the application program does not need to make any additional allowances depending on whether the new renderer requires multiple passes to complete a rendering.

In another aspect of the invention, a graphics rendering system is provided in which more than one renderer can be active simultaneously. In an embodiment, this is accomplished by storing the current state of rendering for each renderer in a respective "view object" (or objects) which the application program specifies when calling the rendering system to draw a three-dimensional object. Again, the re-traverse flag obviates any necessity for the application program to make special allowances if one or more of the simultaneously active renderers require multiple passes to complete a rendering.

In another aspect of the invention, a graphics rendering system is provided which is extensible to support dynamically registered renderers. Again, because of the re-traverse flag, there is no restriction that the newly registered renderers be single or multi-pass renderers.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with respect to particular embodiments thereof and reference will be made to the drawings, in which:

FIG. 1 is a simplified block diagram of a computer system implementing the present invention;

FIG. 2 illustrates a software architecture employing the invention;

FIG. 3 is a flowchart illustrating the overall flow of a program using the invention;

FIG. 4 is a detail of step 308 in FIG. 3;

FIGS. 5, 7 and 9 illustrate object data structures in memory;

FIG. 6 illustrates a class hierarchy used in an embodiment of the invention;

FIG. 8 illustrates a model hierarchy created by an example program using the invention; and

FIG. 10 is a flowchart illustrating the creation of a new object in memory.

DETAILED DESCRIPTION

FIG. 1 is a simplified block diagram of a computer system implementing the present invention. Although certain types of computer architectures might take better advantage of the invention than others, the invention can be implemented on virtually any type of architecture. In the architecture of FIG. 1, a CPU 102, a memory 104, and an I/O subsystem 106 are all connected to a bus 108. The CPU 102 issues signals over the bus 108 for reading and writing to the memory 104 or to the I/O subsystem 106, in order to manipulate data in the manner described herein. The CPU issues such signals in response to software instructions that it obtains from the memory 104. The I/O subsystem 106 may also be capable of issuing signals over the bus 108 in order to access memory 104 in a particular embodiment. The system can also include a graphics coprocessor 110, which can offload from the CPU 102 many of the memory-intensive tasks required for rendering an image. In such situations, the display, illustrated in FIG. 1 as 114, is often driven by the I/O subsystem 106. In other systems, a graphics accelerator 112 is connected to the bus 108 and to the display 114. In these systems, the display buffer is typically held inside the graphics accelerator 112 which can not only write specific attributes (e.g. colors) to specific pixels of the display 114 as requested by CPU 102, but can also draw more complicated primitives on the display 114 under the command of CPU 102.

The invention is implemented in the present embodiment in the form of a set of software tools referred to herein as Escher. These software tools include a set of software procedures and a set of header files which define the variable names and data structures used by the procedures. Escher is provided to an application program developer on a storage medium such as a magnetic or optical disk or disks. In one embodiment, the storage medium contains source code for Escher, while in another embodiment, the storage medium contains compiled object code for Escher. In yet another embodiment, the storage medium contains some source code and some object code for Escher. The application developer compiles an application program with Escher and with one or more renderers and stores the resulting object code on a storage medium. The combined object code is later read into memory 104, either entirely or in an overlaid manner, and executed by the CPU 102.

FIG. 2 illustrates the logical position of Escher in a software architecture. As can be seen, logically, the Escher procedures 202 are disposed between an application program 204 and a plurality of renderers 206, 208 and 210. That is, the application program 204 makes procedure calls to Escher procedures via an application program interface (API), and certain procedures within Escher (specifically, certain renderer invocation procedures 212) make procedure calls to the renderers 206, 208 and 210. The application program 204, when making calls to the renderer invocation procedures 212, specifies which renderer the renderer invocation procedures should use. The renderers, in turn, communicate with other hardware components 214 of the platform, such as a display buffer memory, a graphics coprocessor 110 if present, and/or a graphics accelerator 112, if present. The Escher procedures, in addition to the renderer invocation procedures 212, also include renderer installation procedures 216, model building and editing procedures 218, view building and editing procedures 220, and several other kinds of procedures (not shown) which are not pertinent to an understanding of the invention.

FIG. 3 is a flowchart illustrating the overall flow of an example program which uses the invention. In a step 302, the application program 204 calls an Escher initialization procedure (not shown) which, among other things, installs one or more renderers using the renderer installation procedures 216. One of the advantages of Escher is that a variety of different kinds of renderers can be installed, including renderers which were not available at the time the application program was compiled.

In a step 304, the application program calls the view-building/editing procedures 220 of Escher in order to build one or more "view objects". A view object is an object created by the Escher system according to a predefined data structure, which collects in one place a number of viewing criteria such as a camera position, illumination, a hardware destination ("draw context") into which a two-dimensional image is to be rendered, as well as a choice of renderers, among other things.

In a step 306, the application program makes calls to the model-building/editing procedures 218 of Escher, in order to build one or more models. A model is represented in Escher as a hierarchy of one or more objects, each of which describes a geometry (shape), a material attribute (describing the appearance of a surface), a style (such as filled surfaces, edges only or points only), a transform (describing the relative position, orientation and size of three-dimensional objects with respect to world space), or a group (which merely contains further objects at the next level down in the hierarchy). As the term is used herein, a "model" can constitute only a single object, such as a geometry object, without any hierarchically-defined sub-objects.

In a step 308, the application program calls for the renderer invocation procedures 212 in order to have Escher render one or more of the models created by the application program, to one or more of the views defined by the application program. The API for the rendered invocation procedures 212 includes both immediate-mode calls as well as retained-mode calls. For immediate-mode calls, the application program passes in only non-hierarchical data structures to render. The Escher system passes the structures immediately to the specified renderer, without caching any intermediate results. For material, style and transform objects, the Escher system merely adjusts the current "state" of the view, thereby affecting the rendering of subsequently received geometries. For retained-mode calls to the renderer invocation procedures 212, the object passed by the application program 204 can be an entire hierarchically defined model; the renderer invocation procedures 212 automatically traverse this model, making appropriate calls to the specified renderer at appropriate points in the traversal.

In both immediate-mode calls and retained-mode calls, the application program 204 specifies to the renderer invocation procedures 212 both the object to render and a view object. The current "state" of rendering is always maintained within the data structure of the view object, so the application program can render the same model to more than one view at the same time merely by interspersing calls which specify one view object and calls that specify another view object. These calls will not interfere with each other (unless, of course, both view objects designate the same draw context). The two view objects could specify the same or different renderers, and the draw context specified in the view objects can be for output to the same or different kinds of output devices (such as two different windows on a single display, or one for a display and one for a printer). Moreover, the application program 204 can intermix immediate-mode calls and retained-mode calls for the same view, thereby allowing the application developer to optimize the storage and/or traversal of different parts of a scene differently.

Some of these possibilities are illustrated in an example flowchart shown in FIG. 4. In a step 402, the application program calls the renderer invocation procedures 212 specifying a first model and a first view object. In a step 404, the application program calls the renderer invocation procedures specifying the first model and a second view object. In a step 406, the application program calls the renderer invocation procedures specifying a second model and the first view object. In a step 408, the application program calls the renderer invocation procedures specifying the second model and the second view object. In a step 410, the application program calls the renderer invocation procedures specifying a third model and the first view object, and so on.

The independence of an Escher view object (including identification of renderer) from the model to be rendered, also provides enormous flexibility in the sequence of operations performed by the application program 204. For example, renderers need not be installed (step 302) until just prior the calls to the renderer invocation procedures 212 (step 308). View objects also do not need to be defined or completed (step 304) until after a model is prepared (step 306). Thus an application program might build a model, or part of a model, and allow a user to select a renderer only afterwards. Choice of renderer can be made using, for example, a pop-up browse window which offers a number of already installed renderers, and which also offers the user an ability to install yet another renderer at that time. The application program can then render the model (or models) using the chosen renderer, subsequently edit the model or build new ones, and/or change the choice of renderers, and render the model(s) again, and so on. Such flexibility is made possible because the choice of renderer is not bound up in the model as it is being built, but rather, the application program specifies the renderer in its calls to the renderer invocation procedures 212 of Escher.

A simple C-language application program 204 is set forth in Appendix A hereto. In this example, the program first calls the Escher initialization routine, which installs both a wire frame renderer and a Z-buffer renderer. Next, a view object ("view") is created and a particular renderer (the wire frame renderer) is associated with the view. A camera object and a draw context object are also associated with the view object. The program then creates a model ("group") and adds in a polygon object ("polygon"), a line object ("line"), a transform object ("transform") and a group object ("subGroup"). The application program then adds torus object ("torus", a form of geometry object), to the group object. Next, the model is traversed and rendered using the renderer specified in the view object. The renderer specified in the view object is then changed to the Z-buffer type renderer, and the same model is rendered again using the renderer specified in the view object.

Before continuing, it will be worthwhile to set forth certain naming conventions used in the C-language source code incorporated into the present description. In this description, names that begin with the prefix Et (Escher type) are data types defined in the Escher source code. Names that begin with the prefix Ec (Escher constant) are constants defined in the Escher source code. Names that begin with the prefix Er (Escher routine) are procedure names which are callable by the application program. Names beginning with the prefix Ei (Escher internal) are names of internal Escher procedures which are called only by other Escher procedures. Many of these have counterpart Er procedures which are called by the application program, and which essentially do nothing more than call the corresponding Ei procedure. For this reason, Er and Ei procedure names are used interchangeably herein. Finally, names having the prefix Eg (Escher global) are global variables.

The names of Escher routines begin with Ei or Er, and are followed by subwords which begin with capital letters. The form of most Escher procedure names as used herein is ErFoo₋₋ DoSomething, where Foo is the type of data that the function is to operate on and DoSomething is the operation which the routine is to perform on that kind of data. For example, a procedure to create a new polygon object is named ErPolygon₋₋ New. Other naming conventions will be mentioned as they arise.

The different primary steps of an application program, as illustrated in FIG. 3, will now be described in more detail.

I. RENDERER INSTALLATION

The installation of renderers for Escher uses a generalized extension mechanism which is also used for installing other extensions such as shaders. Extensions for Macintosh implementations are files, stored in mass storage in the hardware of FIG. 1, with a data fork and a resource fork. The data fork contains the code to be loaded by the Escher system, and the resource fork identifies the code fragments in the data fork.

When an application program 204 operating on a Macintosh makes a call to the Escher procedure ErInitialize(), the Escher system looks for all extension files in an extension folder on the computer system. All files that are found and that contain appropriate resource information are then considered available for use by the application program. The extension file specifies an initialization routine, which takes all the necessary steps to "register" the services that it provides with the Escher system, as well as a termination routine.

Escher extensions are loaded into a generalized object hierarchy in the Escher runtime environment. Escher's object hierarchy has an "open" architecture which allows any application to "plug in" a subclass at any of several levels in the hierarchy. Renderers are one of the object classes that may be subclassed.

A. Escher Object System

An object in the Escher system is identified by two handles, namely an object class and an object type. The object class is a pointer of type EtObjectClassPrivate, and the object type is a longword. Because the parent class of each subclass in the Escher class hierarchy provides a certain behavior, Escher stores object private data and object classes in a layered manner. For example, a subclass of the renderer class is abstractly laid out in the manner illustrated in FIG. 5. FIG. 5 shows an object "class" 502, which is a contiguous region of memory containing pointers to all the methods associated with the renderer, in this case a wire frame (WF) renderer. Since the wire frame renderer is subclassed from the renderer class, the renderer class is subclassed from a "shared object" class, and the "shared object" class is subclassed from the generalized "object" class, the method tables 502 first list the methods associated with the object class (region 504). These methods include dispose object, duplicate object and unregister object, among other things. All object class method tables point to the same set of Escher procedures in these entries, unless these entries have been overridden on initialization by one of the descendant classes represented in a particular method table 502. The class 502 next contains a region 506, containing pointers to a set of methods appropriate for all objects in a "shared object" class. As it happens, there are no "shared object" methods, so no space is allocated in this layer. The region 506 is followed by a region 508 containing pointers to a set of methods appropriate for all renderers. There is no layer specifically for the wire frame renderer class because, by convention in the Escher design, leaf classes have no method tables of their own.

Region 512 stores all of the data for an instance of class 502. This region is organized in the same manner as region 502. Specifically, it contains first all of the data which is appropriate to any instance of an object class in region 514, followed by all the data appropriate for any instance of a shared object class in region 516, followed by all the data appropriate for any instance of a renderer object in region 518. Unlike the class data 502, the instance data 512 also contains a region 520 containing all the data appropriate for an instance of a wire frame renderer object. The object data in region 514 contains merely a pointer to the method tables 502, which are common for all instances of wire frame renderer objects. The shared object data in region 516 contains a reference count, and the renderer object data in region 518 and the wire frame renderer data in region 520 are described hereinafter.

Also shown in FIG. 5 for illustrative purposes is a second instance of an object in the wire frame renderer class. The second instance has all its data contained in region 522, in the same format as the data of the first instance in region 512. The object data in region 524 of this data points to the same object class method table 502 as does the object data for the first instance. Note that the second instance in FIG. 5 is provided only to illustrate the relationship between classes and instance data in Escher's object mechanism. It is unlikely that more than one instance of a wire frame renderer in particular would ever coexist in a single instantiation of an application program, but this is not precluded.

At this time, it will be useful to describe the class hierarchy used in the Escher system. This hierarchy is extensive, and only those classes which are pertinent to an understanding of the invention are illustrated in FIG. 6. Referring to FIG. 6, it can be seen that the EtObject class is the parent class to all classes in the hierarchy. The EtSharedObject class is subclassed under EtObject, as are other classes not here pertinent. Subclassed under the EtSharedObject class is an EtShapeObject class, an EtRendererObject class, an EtSetObject class, an EtDrawContext class and an EtViewObject class. Subclassed under the EtShapeObject class are an EtStyleObject class, an EtTransformObject class, an EtCameraObject class, an EtLightObject class, an EtGeometryObject class, an EtGroupObject class, and an EtShaderObject class, and subclassed under the EtRendererObject class are a ZBuffer class and a WireFrame class. Subclassed under the EtStyleObject class is a BackfacingStyle class, among others. Subclassed under the EtTransformObject class are a Rotate class, a Scale class and a Translate class, among others not shown. Subclassed under the EtGeometryObject class are classes for Line, Point, Polygon, PolyLine, Torus and Triangle objects, among others. Subclassed under the EtGroupObject class are an EtLightGroup class, an EtInfoGroup class and an EtDisplayGroup class, the last of which has subclasses EtOrderedGroup and EtListGroup. This class hierarchy describes the storage of method tables and instance data for each of the classes included in the hierarchy. The classes at the ends of the hierarchy (i.e. the "leaves" of the tree structures) are known as "leaf" classes.

B. Registering a Renderer

The basis for object subclassing in Escher is that Escher builds its method tables dynamically at system start-up time using an extensions mechanism. Every object class in the system, including renderer object classes, is "registered" under the control of the ErInitialize procedure called by the application program so that their functionality is available when required. As each extension is loaded, Escher obtains the address of the extension's initialization function from the resource fork of the extension file. It then invokes that function.

The following is a C-language initialization procedure, called ErWF₋₋ Register, used by the wire frame renderer extension.

    ______________________________________                                         Copyright © 1994 Apple Computer, Inc.                                  EtStatus ErWF.sub.-- Register(                                                  void)                                                                          EgWFRendererClass = ErRendererClass.sub.-- Register                           (EcRendererType.sub.-- WireFrame,                                                           "WireFrame",                                                                   ErWF.sub.-- MetaHandler);                                          if (EgWFRendererClass == NULL) {                                                return (EcFailure);                                                           }                                                                              ErRendererClass.sub.-- OverrideGeometryTypeDrawMethod                         (EgWFRendererClass,                                                                         EcGeometryType.sub.-- Polygon,                                                 ErWF.sub.-- Geometry.sub.-- Polygon);                              ErRendererClass.sub.-- OverrideGeometryTypeDrawMethod                         (EgWFRendererClass,                                                                         EcGeometryType.sub.-- Triangle,                                                ErWF.sub.-- Geometry.sub.-- Polygon);                              ErRendererClass.sub.-- OverrideGeometryTypeDrawMethod                         (EgWFRendererClass,                                                                         EcGeometryType.sub.-- PolyLine,                                                ErWF.sub.-- Geometry.sub.-- PolyLine);                             ErRendererClass.sub.-- OverrideGeometryTypeDrawMethod                         (EgWFRendererClass,                                                                         EcGeometryType.sub.-- Line,                                                    ErWF.sub.-- Geometry.sub.-- PolyLine);                             ErRendererClass.sub.-- OverrideGeometryTypeDrawMethod                         (EgWFRendererClass,                                                                         EcGeometryType.sub.-- Point,                                                   ErWF.sub.-- Geometry.sub.-- Point);                                ErRendererClass.sub.-- OverrideGeometryTypeDrawMethod                         (EgWFRendererClass,                                                                         EcGeometryType.sub.-- Marker,                                                  ErWF.sub.-- Geometry.sub.-- Marker);                               ErRendererClass.sub.-- OverrideGeometryTypeDrawMethod                         (EgWFRendererClass,                                                             EcGeometryType.sub.-- Decomposition,                                           ErWF.sub.-- Geometry.sub.-- Decomposition);                                    /* Track transforms and attributes */                                          ErRendererClass.sub.-- OverrideTransformTypeChangedMethod(                    EgWFRendererClass,                                                             EcViewStateType.sub.-- Transform.sub.-- LocalToWorldMatrix,                    ErWF.sub.-- UpdateTransformations);                                             ErRendererClass.sub.-- OverrideAttributeSetChangedMethod(                     EgWFRendererClass,                                                             ErWF.sub.-- UpdateAttributeSet);                                                return (EcSuccess);                                                           }                                                                              ______________________________________                                    

When a class is registered, it supplies methods which determine the behavior of instances of the class. The class supplies these methods to Escher via a metahandler. A metahandler is a function which maps Escher method types to function pointers. Escher asks the metahandler function to give it a method of a certain type, and if the metahandler knows the method type, it returns its corresponding method. If the metahandler does not know the method type being asked for, it returns NULL. As can be seen, the first step of ErWF₋₋ Register registers the new wire frame subclass by calling the renderer class's registration method ErRendererClass₋₋ Register procedure with an identification of the wire frame renderer's metahandler, ErWF₋₋ MetaHandler, as an argument. The wire frame renderer's metahandler is as follows:

    ______________________________________                                         Copyright © 1994 Apple Computer, Inc.                                  static EtFunctionPointer ErWF.sub.-- MetaHandler(                               EtMethodType   methodType)                                                     switch (methodType) {                                                            case EcMethodType.sub.-- NewRenderer:                                            return (EtFunctionPointer) ErWF.sub.-- New;                                  case EcMethodType.sub.-- StartRenderer:                                          return (EtFunctionPointer) ErWF.sub.-- Start;                                case EcMethodType.sub.-- EndRenderer:                                            return (EtFunctionPointer) ErWF.sub.-- End;                                  case EcMethodType.sub.-- CancelRenderer:                                         return (EtFunctionPointer) ErWF.sub.-- Cancel;                               case EcMethodType.sub.-- ObjectDelete:                                           return (EtFunctionPointer) ErWF.sub.-- Delete;                               case EcMethodType.sub.-- ObjectRead:                                             return (EtFunctionPointer) EiWF.sub.-- Read;                                 case EcMethodType.sub.-- ObjectAttach:                                           return (EtFunctionPointer) EiWF.sub.-- Attach;                               case EcMethodType.sub.-- ObjectTraverse:                                         return (EtFunctionPointer) EiWF.sub.-- Traverse;                             case EcMethodType.sub.-- ObjectWrite:                                            return (EtFunitionPointer) EiWF.sub.-- Write;                                default:                                                                         return NULL;                                                               }                                                                             }                                                                              ______________________________________                                    

Escher will call the metahandler once for each entry in its method tabled each time requesting the identification of a different wire frame method.

The EtRendererObject class includes a method table for rendering geometric shapes. Every renderer must provide a method to render at least three basic geometry types: point, line and triangle. The renderer can provide methods for rendering more complex geometry types as well. Thus, after registering a metahandler, the wire frame ErWF₋₋ Register procedure above calls the renderer class procedure ErRendererClass₋₋ OverrideGeometryTypeDrawMethod to establish the geometry draw methods that it supports. As can be seen the wire frame renderer registers procedures for rendering geometries of type polygon, triangle, line, polyline (sequence of connected lines) and points, among others.

The ErWF₋₋ Register procedure also overrides certain transform methods and attribute set methods in a method table of the renderer class.

It can be seen that through the object mechanism of Escher, new renderers can be installed at runtime into the Escher system merely by having them subclass themselves under the EtRendererObject class.

II. BUILDING A VIEW OBJECT A. Data Structures

As mentioned, a view object is a data structure which contains, among other things, camera information, lighting information, traverser or state information as well as an indication of a choice of renderers. A view object is an instance of the class EtViewObject which, as indicated in FIG. 6, is a subclass of the class EtSharedObject, which is itself a subclass under the class EtObject. Accordingly, following the format of FIG. 5, a view object has the format of FIG. 7 in memory. Specifically, a region of memory 702 is allocated to contain pointers to the methods of EtViewObject class, and this region 702 contains pointers 704 to object methods and pointers 706 to shared object methods. The EtViewObject class is a leaf class, so in accordance with the Escher conventions the class omits a method table specifically for view object methods. Note also that in the present embodiment, there are no shared object methods either.

The structure also includes instance data for the view object in region 710 of memory. This region contains instance data specific to the object class in a portion 712 (pointing to the object class 702), instance data specific to the shared object class in a portion 714 (containing a reference count), and instance data specific to the view object class in a portion 716. The view object data is a data structure of type EtViewPrivate which is set out below.

    ______________________________________                                         Copyright © 1994 Apple Computer, Inc.                                  typedef struct EtViewPrivate {                                                 /*                                                                              * Flags                                                                        */                                                                            unsigned int      started : 1;                                                 unsigned int      cancelRendering: 1;                                          unsigned int      viewOwnsRenderer: 1;                                         long              immediateMode;                                               unsigned long     passNumber;                                                  /*                                                                              * Objects that make up the view                                                */                                                                            EtRendererObject  rendererObject;                                              EtRendererObject  saveRestoreRendererObject;                                   struct EtRendererPrivate                                                                         *renderer;                                                   struct EtRendererClass                                                                           *rendererClass;                                              EtQuality         quality;                                                     EtDrawContextObject                                                                              drawContext;                                                 EtCameraObject    camera;                                                      EtGroupObject     lights;                                                      EtShaderObject    atmosphericShader;                                           EtShaderObject    backgroundShader;                                            EtShaderObject    foregroundShader;                                            EtAttributeSet    defaultAttributeSet;                                         EtIdlerCallback   idlerCallback;                                               void              *idlerData;                                                  }   EtViewPrivate;                                                             ______________________________________                                    

As can be seen, a view object includes, among other things, a pointer (*rendererClass) to the methods of a current chosen renderer, a pointer (*renderer) to the instance data of the current renderer, a draw context object, a camera object, lighting objects (lights), and several shader objects.

The EtRendererPrivate structure is defined as follows:

    ______________________________________                                         Copyright © 1994 Apple Computer, Inc.                                  typedef struct EtRendererPrivate {                                             EtViewObject           view;                                                   struct EtAttributeState                                                                               *state;                                                 struct EtViewGroupsPrivate                                                                            *groups;                                                struct EtRendererShadersPrivate                                                                       *shaders;                                               struct EtRendererAttributeSetPrivate                                                                  *attributeSet;                                          struct EtRendererStylesPrivate                                                                        *styles;                                                struct EtRendererTransformersPrivate                                                                  *transforms;                                            struct EtViewStudiosPrivate                                                                           *studios;                                               }   EtRendererPrivate;                                                         ______________________________________                                    

This structure contains pointers to a series of stacks which indicate the current state of a traversal. As described in more detail below, these state stacks are pushed each time Escher's traverser opens a "group" object in a model and begins traversing the next level of the hierarchy. These stacks are popped up to the prior state when the traverser completes its work at all lower levels of the model hierarchy and closes a "group" object.

The EtRendererClass data structure is defined as follows:

    __________________________________________________________________________     Copyright © 1994 Apple Computer, Inc.                                  typedef struct EtRendererClass {                                               EtRendererInfo          rendererInfo;                                          EtRendererNewMethod     newRenderer;                                           EtRendererStartMethod   startRenderer;                                         EtRendererEndMethod     endRenderer;                                           EtRendererCancelMethod  cancelRenderer;                                        EtRendererSyncMethod    syncRenderer;                                          EtRendererPushMethod    pushRenderer;                                          EtRendererPopMethod     popRenderer;                                           EtRendererIsBoundingBoxVisibleMethod                                                                   isBBoxVisible;                                         EtRendererTraceRayMethod                                                                               traceRay;                                              EtRendererStartCacheMethod                                                                             startRenderingCache;                                   EtRendererIsCacheValidMethod                                                                           isRenderingCacheValid;                                 EtRendererClearCacheMethod                                                                             clearRenderingCache;                                   EtMethodTable           *geometryDrawMethods;                                  EtMethodTable           *geometryPickMethods;                                  EtMethodTable           *shaderDrawMethods;                                    EtMethodTable           *shaderGetMethods;                                     EtMethodTable           *shaderChangedMethods;                                 EtMethodTable           *studioDrawMethods;                                    EtMethodTable           *studioGetMethods;                                     EtMethodTable           *styleDrawMethods;                                     EtMethodTable           *styleGetMethods;                                      EtMethodTable           *styleChangedMethods;                                  EtMethodTable           *transformDrawMethods;                                 EtMethodTable           *transformGetMethods;                                  EtMethodTable           *transformChangedMethods;                              EtMethodTable           *groupMethods;                                         EtRendererGeometryDrawMethod                                                                           geometryDrawMethod;                                    EtRendererShaderDrawMethod                                                                             shaderDrawMethod;                                      EtRendererStyleDrawMethod                                                                              styleDrawMethod;                                       EtRendererTransformDrawMethod                                                                          transformDrawMethod;                                   EtRendererAttributeSetDrawMethod                                                                       attributeSetDrawMethod;                                EtRendererAttributeSetGetMethod                                                                        attributeSetGetMethod;                                 EtRendererAttributeSetChangedMethod                                                                    attributeSetChangedMethod;                             EtRendererAttributeDrawMethod                                                                          attributeDrawMethod;                                   EtRendererAttributeGetMethod                                                                           attributeGetMethod;                                    } EtRendererClass;                                                             __________________________________________________________________________

It will be recalled that the initialization procedure of the wire frame renderer set forth above establishes a metahandler which Escher can call to override certain methods of the renderer class. Escher places the pointers returned by the metahandler in the EtRendererClass structure fields defined above for the corresponding methods. It will also be recalled that the wire frame renderer initialization procedure overrides some methods, in particular certain geometry drawing methods, using a ErRendererClass₋₋ OverrideGeometryTypeDrawMethod procedure. This procedure writes the pointer to the specified renderer procedure into the method table pointed to by *geometryDrawMethods, thereby overriding default methods set up by the Escher system originally on initialization. The EtMethodTable data structure is merely a list of pointers; for *geometryDrawMethods, each entry in the table points to the procedure for rendering a corresponding geometry type, for example point, line, triangle, etc. The correspondence between locations in this table and geometry types is fixed at compile time.

One other field in the EtRendererClass data structure which bears mentioning is the geometryDrawMethod field. This field contains a pointer a procedure which Escher will call if it has been asked to draw a geometry type which the current renderer does not support (i.e. the method table entry in *geometryDrawMethods for the geometry is NULL). That procedure decomposes the specified geometry into similar geometries as described in more detail hereinafter. In the present embodiment, the decomposition procedure cannot be overridden. In another embodiment of the invention, however, a renderer can override this decomposition method in order to optimize the process.

B. Procedures

The process of building a view object in step 304 (FIG. 3) is basically the process of writing desired information into the view object data structure. The example application program in Appendix A will be used to illustrate the process.

1. Creating a View Object

After initialization, the application program creates a new view object by calling view=ErView₋₋ New(). This procedure merely creates a new instance of an object in the EtViewObject class and fills it with default data.

2. Setting the Renderer

A renderer can be attached to a view object by calling an Escher procedure ErView₋₋ SetRenderer and passing in the view object and the renderer object. This call will increment the reference count of the renderer passed in. If a renderer object is already set, its reference count is decremented.

A renderer can also be set by renderer type without first having to obtain an instance of a renderer object. In this case, the application program calls the Escher routine ErView₋₋ SetRendererByType, and this is the procedure which is called by the example program in Appendix A. The parameters passed to this procedure are the view object and a type designation, which is a four-character code designating a type of renderer (for example, wire frame or Z-buffer). The Escher procedure ErView₋₋ SetRendererByType determines what renderer has been registered of the type specified and if such a renderer has been registered, writes a pointer to the renderer instance data into the appropriate entry of the specified view object.

3. Setting the Camera

Before setting the camera, a camera object must be created and initialized. The example program in Appendix A accomplishes this by writing the camera perspective data into an appropriate perspectiveData data structure and assigning it to a camera object using the Escher procedure ErViewAngleAspectCamera₋₋ NewData. Once a camera object is obtained, it can be associated with the view object by calling ErView₋₋ SetCamera passing in the view object and the camera object. This call will increment the reference count of the camera object passed in. If the camera object was already set, its reference count will be decremented. The example program in Appendix A then disposes of the camera object using the Escher function ErObject₋₋ Dispose since the camera object is no longer separately needed.

4. Setting the Drawing Context

Before setting the draw context, a draw context object must be created and initialized. In the example program in Appendix A, this is accomplished by setting up an appropriate data structure pixmapData and passing it to the Escher procedure ErPixmapDrawContext₋₋ NewData. The draw context object is then associated with the view object using the call ErView₋₋ SetDrawContext and passing in the view object and the draw context object. The application program example in Appendix A then disposes of the draw context object by passing it to the Escher function ErObject₋₋ Dispose.

Other characteristics of the view can also be specified in a similar manner, such as lighting and shaders.

It can be seen that the building and editing of one view object is entirely separate from the building and editing of another view object, so more than one view object (including those which specify the same or different renderers) can coexist without interfering with each other.

III. BUILDING A MODEL

The modeling paradigm used in Escher can be best understood by comparison to the modeling paradigms used in two existing products available from Apple Computer: QuickDraw and QuickDraw GX. QuickDraw GX is described in Apple Computer, "QuickDraw GX Programmer's Overview" (1994), incorporated herein by reference. Both QuickDraw and QuickDraw GX perform two-dimensional graphics processing rather than three-dimensional processing. The QuickDraw two dimensional graphics system features a procedural interface and a global graphics state which defines the color, transfer mode and pattern of the shapes that it draws. When the QuickDraw shape drawing routines are called, QuickDraw draws the shape according to the variables in its graphics state. An application program can manipulate the graphics state through the use of other calls to QuickDraw.

QuickDraw GX differs from QuickDraw in that rather than a procedural interface with a system-maintained graphics state, shapes are represented as objects that encapsulate all information needed to draw them. There is no system-maintained graphics state. Because shapes are objects, QuickDraw GX can provide utilities to operate on such shapes that QuickDraw cannot, because QuickDraw's routines only draw images to a pixel map. QuickDrawGX provides functionality to operate on shapes, such as "hit" testing and geometric intersection.

The main data type in QuickDraw GX is a "shape", which encapsulates geometry and other drawing information. Shapes are drawn through a "view port" which transforms the shapes into "view device" coordinates. When a shape is passed to QuickDraw GX for drawing, the shape is drawn through each view port that is attached to the shape. A view port may overlap several view devices, in which case the shape is drawn to each view device at its correct resolution. QuickDraw GX shapes can be organized into hierarchies through the use of the "picture shape". Each shape has a "type" associated with it that indicates whether it is a line, polygon, curve, etc. Access to shapes is through a procedural interface and handle.

QuickDraw GX's shape drawing procedure can be called at any time to draw a shape. The ordering of shapes being drawn on top of each other is dependent on the order in which they are drawn. No state information is retained between drawing commands, except for internal caching information, since each shape encapsulates all the information needed to draw that shape including the view port, view device, color, transfer mode and drawing style.

The Escher system differs significantly from QuickDraw and QuickDraw GX due to the nature of three-dimensional rendering algorithms and the typically larger volumes of data required to describe a three-dimensional model.

Escher shapes do not encapsulate all information required to draw themselves. Ratherr Escher maintains a "state" that provides additional information on how a shape is to be rendered, much as the original QuickDraw maintains a drawing state for the foreground color in the graphics port. Escher's state mechanism allows hierarchical models to be built which specify information such as color or drawing style that is inherited by shapes at lower levels of the hierarchy. The mechanism provides increased flexibility in instancing a model to be used several times in a scene but with varying attributes without respecifying the geometry. Escher also differs from QuickDraw GX in that the drawing area for rendering is specified in Escher prior to the rendering of the image, through attachment to a view object, rather than being attached to every shape as in QuickDraw GX.

Escher provides several data types to encapsulate geometry and appearance in a model hierarchy. The general classes of data types are geometry, "attribute set", style, transform and group. It is advantageous to separate appearance into several types of data in this manner due to the typical complexity of three-dimensional models. Even simple three-dimensional shapes may require hundreds or thousands of geometric shapes in order to produce any semblance of realism. To create a realistic-appearing scene of any complexity, such as a room in a house, may require hundreds of thousands or even millions of geometric shapes. When dealing with models of this magnitude, it is often advantageous to apply a single appearance characteristic to a large group of geometric shapes, thereby saving memory and simplifying the building and editing of such a model. The above data types facilitate these goals.

In QuickDraw GX model hierarchies, transform mappings of a "picture shape" are considered concatenated with those of the shapes that they encompass. This provides a means of referencing the same shape more than once, and using transform mappings to move or transform the shape without having a second copy of it stored in memory. Such concatenation is accomplished in QuickDraw GX through a state mechanism which keeps a record of the "current transformation mapping" as a model hierarchy is traversed. QuickDraw GX picture hierarchies are traversed from top to bottom, left to right. As shapes lower in the hierarchy are traversed, their transform mappings are concatenated with the current transform mapping. After all shapes inside a picture shape have been drawn, the traversal returns to the previous picture, if any, and resumes traversal of its shapes. This is called "top-down traversal". When returning from a traversal, the current transformation mapping is restored to what it was before the picture shape was entered. This mechanism can be thought of as stack of mappings that is pushed and popped during traversal. When the root picture shape is finished drawing, the current mapping is NULL.

Escher provides a similar traversal mechanism, although due to increased complexity of three-dimensional models, more than merely a transform mapping is inherited. Appearances and styles are inherited as well.

Whereas in QuickDraw GX, there is no "current transform mapping" unless the QuickDraw GX system is traversing a picture, in Escher it is possible for an application program to push and pop the current state itself. In fact, application programs can maintain Escher shapes in an application-specific hierarchical data structure, if desired, and through careful sequencing of calls to the Escher procedures 202, can simulate a system hierarchy. This feature is extremely advantageous for application programs that use complex application-specific hierarchical data structures, such as animation systems, or when porting an existing application program that already has its own data structures. The PHIGS prior art system and other retained-mode systems require that the entire model reside in a single hierarchy, and GL treats each object independently. Escher, on the other hand, allows a mixture. Art application program can set the state, draw a number of independent geometric objects, then have the Escher system render a model built and stored in the Escher system's data structure, then draw more independent geometric objects, and so on.

FIG. 8 is a graph of a simple model hierarchy which is built by the example application program in Appendix A. It includes two levels, the first (root) level encapsulated by the EtDisplayGroupObject called "group" and the second encapsulated by the EtDisplayGroupObject called "subGroup". Each EtDisplayGroupObject can be thought of as a node in the hierarchy. Each node can have associated therewith style objects, geometry objects, transform objects and other group objects (as in FIG. 8), as well as shader objects and attributes set objects.

Escher supports two kinds of EtDisplayGroupObjects, namely those in the subclass EtOrderedGroup and EtListGroup. Objects attached to an instance in the EtListGroup class have no order except the order in which they were added to the group. During traversal, when Escher encounters a list group object, each object in the list is processed ("executed") in the sequence in which it was added to the group originally. Referring to FIG. 8, once the group group is opened, during a traversal, Escher will execute backfacingStyle, then polygons then lines then transform, then subGroup, in that sequence, since that was the sequence with which they were added to group. Thus the changes to the rendering state which are caused by backfacingStyle will apply to both polygon and line, whereas the changes to the rendering state caused by transform will apply only to the objects in subGroup. The Escher API includes calls which permit the application program to add objects to the beginning of the list, to the end of the list, or between objects already on the list.

A group object of subclass EtOrderedGroup is similar to a list group object, except that the traverser sorts the object attached to a group according to type before they are executed. In particular, objects are executed in the following sequence: transform objects, style objects, attribute set objects, shaders, geometries and additional groups.

For both kinds of group objects, when a subgroup is opened, the then-current state of the renderer is inherited. Objects in the subgroup can change any characteristic of the state for subsequently executed objects in the subgroup (or in subgroups of the subgroup), but upon return to the parent group, the state is restored to its condition before the traverser entered the subgroup.

Group objects also have a "state" associated with them, although this is not to be confused with the "state" of the traverser. The state of a group is merely a collection of flags that define aspects of how the group is to behave. Most of the flags are not important for an understanding of the present invention, but it may be helpful to understand one such flag, namely "in-line".

In modeling applications, it sometimes can be useful to group a set of materials or styles together into a bundle that can be referenced several times in a model. However, if such a bundle is created using the normal pushing and popping of traverser state as described above, these objects will not have the desired effect on the model as the group will pop the state after it is executed by the traverser. Accordingly, the application program can set the "in-line" flag of the group object, thereby specifying that entry to the group and exit from the group are not to push or pop the state of the traverser. The Escher API provides procedure calls to set, clear and get the current value of this flag.

A. Data Structures

As with the procedure for building a view, it will be helpful to described certain data structures before describing the Escher procedures which an application program can call to build a model.

In the example application program in Appendix A, the groups group and subGroup are ordered display group objects. They have a type EtDisplayGroup, which in the class hierarchy of FIG. 6, has a class ancestry of EtGroupObject, EtShapeObject, EtSharedObject, and ultimately EtObject. Accordingly, they are represented in memory with data structures as shown in FIG. 9. Specifically, the ordered display group class method table is contained in a block of memory 902, and the instance data for group and subGroup are contained in blocks 904 and 906, respectively. The ordered display group class 902 begins with the region 908 containing pointers to object methods, similarly to region 504 in FIG. 5. Region 908 is followed by region 910, which contains pointers to all methods specific to shared objects (there are none in the present embodiment). This is followed by a region 912, containing pointers to all shape object methods, and this is followed by a region 914 containing pointers to the methods specific to group objects. The last -mentioned data structure, EtGroupClass, has the following typedef:

    ______________________________________                                         Copyright © 1994 Apple Computer, Inc.                                  typedef struct EtGroupClass {                                                  EtGroupAcceptObjectMethod                                                                           accept;                                                   EtGroupGetObjectListMethod                                                                          getObjectList;                                            EtGroupCountObjectsMethod                                                                           countObjects;                                             EtGroupAddObjectMethod                                                                              add;                                                      EtGroupAddObjectBeforeMethod                                                                        addBefore;                                                RtGroupAddObjectAfterMethod                                                                         addAfter;                                                 EtGroupRemoveObjectMethod                                                                           remove;                                                   EtGroupEmptyMethod   empty;                                                    }   EtGroupClass;                                                              ______________________________________                                    

As can be seen, it includes entries for pointers to several methods, including, among other things, a method to add an object to the end of the group (add), and methods to add an object at a specific location within the list of objects already assigned to a group (addBefore and addAfter).

After region 914, the ordered display group class method table 902 contains pointers to the methods specific to display groups in a region 916. This data structure, EtDisplayGroupClass, is defined as follows:

    ______________________________________                                         Copyright © 1994 Apple Computer, Inc.                                  typedef struct EtDisplayGroupClass {                                           long              displayGroupUniqueNumber;                                    EtBoundingBoxMethod                                                                              boundingBox;                                                 EtBoundingSphereMethod                                                                           boundingSphere;                                              EtDisplayGroupPickMethod                                                                         pick;                                                        }   EtDisplayGroupClass;                                                       ______________________________________                                    

The methods in the EtDisplayGroupClass are not pertinent to an understanding of the invention, except to note that the class does not contain pointers to any drawing methods. As previously explained, these methods are identified in a view object rather than a group object, so that they can be overridden by a renderer. Any drawing method which is identified in the class for an object would become part of the model being built, would become intertwined with the model itself, and would be difficult to change when an application program desires to render the model using a different renderer. Such an arrangement would also make it difficult to have more than one renderer active simultaneously as previously described.

Referring again to the method table 902, as is typical of the design of the Escher system with respect to leaf classes in the class hierarchy (FIG. 6), the table contains no methods specific to ordered display groups.

The data structure of the instance data for subGroup is the same for that of group, so only the data structure for group will be described. As with the data structure of instance data block 512 in FIG. 5, the block 904 begins with object data in region 920, specifically a pointer to the ordered display group class block 902. This is followed by a region 922, containing the shared object data, specifically a reference count. This is followed by a region 924, containing the shape object data (which is also very short). The shape object data region 924 is followed by a region 926 containing the object data for group objects, of which there is none in the present embodiment.

The region 926 is followed by a region 928 containing the instance data for display group objects. Such data includes only the "state" flags described above. Region 928 is followed by a region 930, which contains the data specific to an instance of an ordered display group object. This data has a format defined as follows, where EtDLList is a typedef for a doubly linked list:

    ______________________________________                                         Copyright © 1994 Apple Computer, Inc.                                  typedef struct EtOrderedDisplayGroupPrivate {                                  EtDLList          *transforms;                                                 EtDLList          *styles;                                                     EtDLList          *attributeSet;                                               EtDLList          *shaders;                                                    EtDLList          *geometries;                                                 EtDLList          *groups;                                                     }   EtOrderedDisplayGroupPrivate;                                              ______________________________________                                    

As can be seen, the region 930 contains pointers to six doubly linked lists, one for each of the types of objects which can be added to a display group object. For completeness, note that if group were a list display group rather than an ordered display group, the only difference would be the structure of region 930. Specifically, region 930, which would have a structure defined as EtListDisplayGroupPrivate, would contain only a pointer to a single doubly linked list for all the objects which are added to the display group.

In addition to the EtDisplayGroupObject structure, the example application program in Appendix A also uses an EtStyleObject data structure, an EtGeometryOject data structure, and an EtTransformObject data structure. These structures all refer to classes of objects which, like EtGroupObject are subclassed from the EtShapeObject class (see FIG. 6). As with other objects described herein, they each refer to a method table like 902 in FIG. 9, and contain private instance data, like region 904 or 906 in FIG. 9. These three classes are each subclassed from the same parent class (EtShapeObject) as is EtGroupObject (see FIG. 6), and therefore the first three layers of both the class data and the instance data will have the same structure as shown in FIG. 9. The leaf class (BackfacingStyle) actually used in the example program in the EtStyleObject class is a subclass of the EtStyleObject class, so the fourth and last layer of the class methods for that object will contain pointers to the methods specific to style objects. The fourth layer of the instance data contains private data appropriate to all style objects, if any, and the fifth and last layer contains data appropriate to backfacing style objects.

Similarly, the leaf class (translate) actually used in the example program in the EtTransformObject class is a subclass of the EtTransformObject class (see FIG. 6), so the fourth and last layer of the class methods for the transform object contains pointers to methods specifically for transform objects. The fourth layer of instance data for the transform object contains data specific to transform objects, and the fifth and last layer contains data specific to translate transform objects.

The example program creates three objects (polygon, line, torus) which are geometry objects, and all of them are in leaf classes which are subclasses of the EtGeometryObject class (see FIG. 6). Thus the fourth and last layer of the class method table for each of these objects contains pointers to methods specifically for geometry objects. The fourth layer of the instance data for each of these objects contains data specifically for any geometry object, and the fifth and last layer of the instance data for each of these objects contains data specifically for polygon, line and torus objects, respectively.

In Escher, whereas attribute sets (containing attributes such as diffuse color) define surface appearance characteristics, styles tell a renderer how to draw a geometric shape. For example, a polygon can be rendered as a solid filled shape, or with its edges only. Another example is that surfaces can be rendered smoothly or with a faceted appearance. Yet another example, indicated by a backfacing style object (subclass of EtStyleObject), determines whether or not shapes that face away from the camera are to be displayed. EcBackfacingStyle₋₋ RemoveBackfacing, the characteristic used in the example program in Appendix A, specifies that shapes that face away from the camera are not to be drawn. The private data for a backfacing style object contains a longword which contains a constant indicating whether both front and backfacing surfaces are to be drawn, whether backfacing surfaces are to be removed, or whether backfacing surfaces that do not have two-sided attributes are to have their normals flipped so that they always face toward the camera. An application program specifies style characteristics in a model by creating an appropriate style object and adding it at a desired position in a group object of the model.

For geometry objects, the private data in the last layer of instance data contains information necessary to describe a particular geometry. For example, the private data in the last layer of a polygon object specifies the number of vertices, the vertices and certain attributes not here relevant. The private data in the last layer of a line object contains the locations of the two end points of the line, as well as some attribute data, and the private data in the last layer of a torus object contains an origin, an orientation, a major radius and a minor radius, as well as some attribute data.

A transform object allows the coordinate system containing geometric shapes to be changed, thereby allowing objects to be positioned and oriented in space. Transforms are useful because they do not alter the geometric representation of objects (the vertices or other data that describe the shape), rather they are applied as matrices at rendering time, temporarily "moving" an object in space. This allows a single object to be referenced multiple times with different transformations in a model, providing the ability to have an object placed in many different locations within a scene. An application program specifies a transform by creating an appropriate transform object, from a subclass of the EtTransformObject class (FIG. 6), and adding it at an appropriate position in an appropriate group in a model. A transform specified by an application program remains in the form specified until the time of rendering, at which point Escher converts the transformation to a temporary matrix that is applied to subsequent objects in the group. Matrices are pre-multiplied to vectors of an object. Escher transformations pre-multiply the current transformation matrix; therefore, application programs specify transformations that are to be concatenated in reverse order. This is consistent with the application of matrices in a hierarchy. That is, matrices that are specified at the top of the hierarchy are applied last, and matrices specified just prior to an object are applied first.

Escher supports a number of different kinds of transforms, each of which is specified to Escher by using an object from a respective subclass of EtTransformObject. Three such subclasses are illustrated in FIG. 6 (Rotate, Scale and Translate). The Escher API provides procedures for creating and disposing of transform objects, drawing them in immediate mode, setting their contents to new data, getting the private data of a transform object, writing a transform object to a file, and so on. The private data in the lowest layer of instance data for a translate transform object, which is the leaf class used in the example program in Appendix A, specifies the x, y and z coordinates of translation.

B. Procedures

1. ErOrderedDisplayGroup₋₋ New()

Referring to Appendix A and FIG. 8, the example application program begins building the model of group 802 by first creating the new ordered display group object group, using the procedure ErOrderedDisplayGroup₋₋ New(). Escher's object creation mechanism is a recursive mechanism which is illustrated generally in the flowchart of FIG. 10. Note that the class method table for the ordered display group class was created during initialization upon registration of the ordered display group class, so only a block of instance data needs to be created and initialized in the new object mechanism.

Referring to FIG. 10, first, in a step 1002, the leaf class's `New` procedure calls its parent class `New` procedure with: (a) the static variable which, when the leaf class was registered, points to the top of that leaf class's method tabled and (b) the size of the leaf class's Private data structure. In a step 1004, each higher class's `New` procedure then calls its respective parent class's `New` procedure with: (a) the object class pointer passed to it, and (b) the size needed by all descendent classes so far plus the size needed by the current class's private data structure. In a step 1006, the ultimate parent class's `New` procedure (EiObject₋₋ New()) (a) allocates memory space for the private data structures needed for its own Private data plus the Private data for all descendent classes. This space will contain the entire block of instance data for the newly created object. It then (b) initializes its own Private data structure (by writing the Object Class pointer passed to it into the top of the newly allocated memory block), and (c) returns to its caller (which is the next lower class's `New` procedure), returning a pointer to the newly allocated memory block. In a step 1008, each lower classes `New` procedure then: (a) calls its own Get procedure to get a pointer to the current class's own private data structure within the newly allocated instance data block; (b) initializes its own Private data structure; and (c) returns to its caller (which is the next lower class's `New` procedure), returning a pointer to the newly allocated memory block. The Leaf class's `New` procedure does the same, except that the caller is typically the application program rather than a subclass's `New` procedure.

A few sample ones of Escher's `New` procedures will illustrate better how this is achieved. The `New` procedure for an ordered display group object, which is the routine called by the application program, is as follows:

    __________________________________________________________________________     Copyright © 1994 Apple Computer, Inc.                                  EtGroupObject EiOrderedDisplayGroup.sub.-- New(                                void)                                                                          EtGroupObject     group;                                                       EtOrderedDisplayGroupPrivate                                                                     *ordered;                                                    group = EiDisplayGroup.sub.-- New(EgOrderedDisplayGroupClass,                                sizeof(EtOrderedDisplayGroupPrivate));                           if (!group) {                                                                   return (NULL);                                                                }                                                                              ordered = EiOrderedDisplayGroup.sub.-- GetOrderedDisplayGroup(group);          ordered->transforms                                                                            =                                                              ordered->styles =                                                              ordered->attributeSet                                                                          =                                                              ordered->shaders                                                                               =                                                              ordered->geometries                                                                            =                                                              ordered->groups = NULL;                                                        return (group);                                                                }                                                                              __________________________________________________________________________

As can be seen this routine first calls the `New` procedure of its parent class, EiDisplayGroup₋₋ New() with the pointer to the ordered display group class method tables EgOrderedDisplayGroupClass and the size of the private data structure EtOrderedDisplayGroupprivate needed in the lowest layer of the instance data for the object being created. When that procedure returns, the block of instance data has been allocated and higher levels have been initialized. After some error checking, the EiOrderedDisplayGroup₋₋ New() procedure gets a pointer (ordered) to the ordered display group private data structure of the allocated block, and initializes all of the doubly linked lists to NULL. The procedure then returns to the application program, returning the pointer to the newly created instance data block.

The `New` procedure of the display group class is as follows:

    __________________________________________________________________________     Copyright © 1994 Apple Computer, Inc.                                  EtGroupObject EiDisplayGroup.sub.-- New(                                       EtObjectClass                                                                              objectClass,                                                       unsigned long                                                                              dataSize)                                                          EtGroupObject newGroup;                                                        EtDisplayGroupPrivate                                                                        *groupPrivate;                                                   EiAssert(objectClass);                                                         newGroup = EiGroup.sub.-- New(objectClass,                                                   sizeof(EtDisplayGroupPrivate) + dataSize);                       if (newGroup == NULL) {                                                         return (NULL);                                                                }                                                                              groupPrivate = EiDisplayGroup.sub.-- GetDisplayGroup(newGroup);                EiAssert(groupPrivate);                                                        groupPrivate->state = EcDisplayGroupStateMask.sub.-- IsDrawn                                                     |                                             EcDisplayGroupStateMask.sub.-- UseBoundingBox                                                          |                                             EcDisplayGroupStateMask.sub.-- UseBoundingSphere                                                       |                                             EcDisplayGroupStateMask.sub.-- IsPicked                                                                &                                                      (˜EcDisplayGroupStateMask.sub.-- IsInline);                    return (new Group);                                                            }                                                                              __________________________________________________________________________

As can be seen, this procedure first calls its own parent class's `New` procedure EiGroup₋₋ New, passing the pointer to the object class and a data size given by the size of private data needed by the parent class plus the amount of memory space needed for the private data of objects in the group class (EtDisplayGroupPrivate). After error checking, EiDisplayGroup₋₋ New () gets a pointer groupPrivate to the group object's private data and initializes it. The procedure then returns to its caller, EiOrderedDisplayGroup₋₋ New ().

The `New` procedure in the EtGroupObject class is as follows:

    ______________________________________                                         Copyright © 1994 Apple Computer, Inc.                                  EtGroupObject EiGroup.sub.-- New(                                              EtObjectClass objectClass,                                                     unsigned long dataSize)                                                        EtGroupObject newGroup;                                                        EiAssert(objectClass);                                                         newGroup = EiShape.sub.-- New(objectClass, NULL, dataSize);                    if (newGroup == NULL)                                                          {                                                                              return NULL;                                                                   }                                                                              return newGroup;                                                               }                                                                              ______________________________________                                    

The above procedure follows the same outline as the `New` procedures for the display group class and the ordered display group class, with the variation that there is no private data specifically for the class EtGroupObject. Thus the EiGroup₋₋ New() procedure does not initialize any private data, and the data size which it passes to the `New` procedure of the parent class, EiShape₋₋ New(), is the same as the data size which is passed by the display group class's `New` procedure to EiGroup₋₋ New().

The procedures continue up through the `New` procedure of the EtObject class. It will be appreciated that Escher uses the same recursive mechanism to create objects in every class of the class hierarchy. Moreover, it will be appreciated that Escher uses similar recursive techniques to perform the functions of a large number of the Escher API calls.

2. ErGroup₋₋ AddObject()

Referring again to the example application program in Appendix A, after a new ordered display group object group is created, the program creates a backfacing style object and adds it to the group. This part of the example program is not important for an understanding of the invention, but rather is included merely to illustrate that such an object can be added to a group.

Next, the example program creates a polygon object polygon using the Escher procedure ErPolygon₋₋ New(). This procedure operates similarly to the `New` procedures described above with respect to the creation of an ordered display group object, and need not be described again. The example program then adds polygon to group using the Escher API call ErGroup₋₋ AddObject(). The latter procedure is as follows:

    __________________________________________________________________________     Copyright © 1994 Apple Computer, Inc.                                  EtGroupPosition EiGroup.sub.-- AddObject(                                      EtGroupObject                                                                              group,                                                             EtSharedObject                                                                             object)                                                            EtGroupClass                                                                               *groupClass;                                                       EiAssert(group);                                                               EiAssert(object);                                                              if (EiGroup.sub.-- AcceptObject (group, object) == EcObjectType.sub.--         Invalid)                                                                       {                                                                              EiError.sub.-- Post(EcError.sub.-- InvalidObjectForGroup, NULL);               return NULL;                                                                   }                                                                              groupClass = EiShape.sub.-- GetSubClassMethods(group,                                              EcShapeType.sub.-- Group);                                 EiAssert(groupClass);                                                          EiAssert(groupClass->add);                                                     return (*groupClass->add) (group, object);                                     }                                                                              __________________________________________________________________________

As can be seen, this procedure receives as arguments the object to add to the group, and the group to which it is to be added. After some bookkeeping operations, the procedure calls EiGroup₋₋ AcceptObject(), an Escher procedure, in order to determine whether the specified object is of a type which can be added to the specified kind of group object. For example, if the specified object is a light object, it cannot be added to an ordered display group. In the present case, the result is valid, since a polygon can be added to an EtOrderedDisplayGroup object.

The procedure then obtains a pointer groupclass to the EtGroupObject class method table for the specified ordered display group object, by calling the "get subclass methods" procedure of EtGroupObject's parent class, EtShapeObject. The procedure than calls the "add object" procedure pointed to in that method table and returns to the application program.

The pointer in the method table to the "add object" procedure for the specified kind of group object was written there during initialization when the EtOrderedGroup class registered itself. The specified "add object" procedure is as follows:

    __________________________________________________________________________     Copyright © 1994 Apple Computer, Inc.                                  EtGroupPosition EiOrderedDisplayGroup.sub.-- AddObject(                        EtGroupObject                                                                              group,                                                             EtObject    object)                                                            EtOrderedDisplayGroupPrivate                                                                     *groupData;                                                  EtGroupPosition   newPosition;                                                 EtDLList          *theList;                                                    EtObjectType      objectType;                                                  EiAssert(group);                                                               EiAssert(object);                                                              groupData = EiOrderedDisplayGroup.sub.-- GetOrderedDisplayGroup(group);        EiAssert(groupData);                                                           objectType = EiGroup.sub.-- AcceptObject(group, object);                       if (objectType == EcObjectType.sub.-- Invalid) {                               EiError.sub.-- Post(EcError.sub.-- InvalidObjectForGroup,                              EiWarning.sub.-- Message(                                                        "Invalid object for ordered group: %s",                                        EiObjectClass.sub.-- GetName(EiObject.sub.-- GetClass(object))))               ;                                                                    return (NULL);                                                                 }                                                                              theList = EiOrderedDisplayGroup.sub.-- GetObjectList(groupData,                                  objectType,                                                                    EcTrue);                                                     if (theList == NULL) {                                                         return (NULL);                                                                 }                                                                              newPosition = EiGroupPosition.sub.-- New(group, object);                       if (newPosition == NULL) {                                                     return (NULL);                                                                 }                                                                              EiDLList.sub.-- InsertNodeLast(theList, (EtDLListNode *)newPosition);          return (newPosition);                                                          }                                                                              __________________________________________________________________________

Referring to this procedure, it can be seen that it first uses the ordered display group class's "get" procedure to obtain a pointer groupData to the private data of the specified ordered display group object. The procedure then obtains the type of the specified object to add (which is a geometry object) and, after some error checking, calls EiOrderedDisplayGroup₋₋ GetObjectList() to obtain a pointer, theList, to the particular doubly linked list of the ordered display group object for geometry objects. The procedure calls EiGroupPosition₋₋ New() to create a new list "position" object, and calls EiDLList₋₋ InsertNodeLast () to insert the new "position" object at the end of the doubly linked list. The procedure then returns to its caller, EiGroup₋₋ AddObject().

For completeness, it will be appreciated that the procedure for adding an object to a list display group object is very similar to that for adding an object to an ordered display group object, except that there is only one doubly linked list in a list display group object. It is therefore unnecessary to determine one of six lists to which the object is to be added. It will also be appreciated that in addition to an add-object procedure, Escher's API also includes procedures to add an object after a specified position in the list of the group, add an object before a specified position in a list of the group, remove an object from a specified position in a list of the group, iterate forward and backward through a list of the group, as well as other procedures.

3. ErObject₋₋ Dispose()

After adding an object to the group, the example program in Appendix A "disposes" of the polygon object since it is no longer needed in the application program (apart from its presence in the model being created). So far as the application program is concerned, the polygon object has been deleted. However, in actuality, the Escher ErObject₋₋ Dispose() procedure does not at this time delete the polygon object and deallocate its memory space. Instead, since polygon is a shared object, Escher merely decrements the reference count in the shared object private data for the polygon object. The reference count was set to 1 when polygon was created, and was incremented by 1 when it was added to group. Thus the application program's called to ErObject₋₋ Dispose() merely decrements the reference count from 2 to 1. If the decrementing of the reference count reduces it to 0, then Escher actually deletes the object and deallocates its memory space.

4. New Line Object

The example program in Appendix A next creates a new line object line, adds it to the ordered display group object group, and then "disposes" of the line object line. The Escher procedure calls to accomplish this are similar to those described above with respect to the polygon object polygon, and need not be repeated here.

5. Adding a Transform Object

After placing a style object and two geometry objects into the ordered display group group, the example program in Appendix A creates a translate transform object and adds it to the group. The transform object transform is created by calling the Escher procedure ErTranslateTransform₋₋ New(), which operates in a recursive manner similarly to ErOrderedDisplayGroup₋₋ New () as described above. The example program's subsequent calls to ErGroup₋₋ AddObject(group, transformm) and ErObject₋₋ Dispose (transform) operate as described above. Note that although the example program adds the transform object to the ordered display group object after it has already added two geometry objects, the nature of an ordered display group calls for transform objects to be executed prior to geometry objects. Escher ensures this characteristic by placing the transform object on a separate doubly linked list within the ordered display group object group exclusively for transforms, and placing the geometry objects in a doubly linked list exclusively for geometry objects. Upon rendering, as described hereinafter, Escher will traverse the transform object list before it traverses the geometry object list.

6. Creation and Addition SubGroup

After adding the transform object to the ordered display group object group, the example program in Appendix A creates a new ordered display group subGroup by calling ErOrderedDisplayGroup₋₋ New(). This procedure is described above. The example program then adds subGroup to the previously created ordered display group object group, using the ErGroup₋₋ AddObject() Escher procedure also described above. The example program has thus now created the hierarchy illustrated in FIG. 8. The example program then creates a new geometry object, specifically a torus geometry object torus using an Escher procedure ErTorus₋₋ New(), which operates similarly to ErPolygon₋₋ New() described above. It then adds torus to subGroup using ErGroup₋₋ AddObject(), and disposes of both torus and subGroup using ErObject₋₋ Dispose(). At this point, the entire model illustrated in FIG. 8 has been built and the example program moves on to step 308 (FIG. 3), rendering the model to the view.

IV. RENDERING THE MODEL TO THE VIEWS

In Escher, the rendering of objects to a view takes place between calls to ErView₋₋ StartRendering() and ErView₋₋ EndRendering(). These procedures merely initialize relevant data structures prior to rendering (including pushing an initial state onto a traversal stack) and clean up various bookkeeping information after rendering, respectively. They also include calls to the renderer's own start and end procedures, so that the renderer can do the same. The renderer's start and end procedures were specified to Escher upon registration of the renderer and are identified in appropriate method tables. Specifically, the renderer's end-rendering procedure was returned by the renderer's metahandler in response to being called with the EcMethodType₋₋ EndRenderer constant.

Escher supports multi-pass rendering in which Escher traverses the model more than once, calling appropriate renderer procedures each time. Escher indicates that a re-traverse is required by returning a status flag, EcViewStatus₋₋ ReTraverse, from the ErView₋₋ EndRendering() procedure. When the application program calls ErView₋₋ EndRendering(), that procedure in turn calls the renderer's end-rendering procedure. In the case of the wireframe renderer, this procedure is called ErWF₋₋ End(). The renderer returns the re-traverse flag to Escher's ErView₋₋ EndRendering () procedure, which in turn returns the same to the application program as EcViewStatus₋₋ ReTraverse. Thus the preferred technique is for the application program to place the model drawing calls inside a do loop which repeats for as long as ErView₋₋ EndRendering() returns EcViewStatus₋₋ ReTraverse. This is the format used in the example program in Appendix A.

Note that the calls to ErView₋₋ StartRendering() and ErView₋₋ EndRendering() take a view object view as an argument. An application program can call these procedures in any sequence, specifying different view objects, as long as the call to ErView₋₋ EndRendering() for a particular view object is subsequent the call to ErView₋₋ StartRendering() for the same view object, and all drawing calls to that view are in between. It is also an error to call ErView₋₋ StartRendering() twice for a particular view object without calling ErView-EndRendering() for the same view object in between, and it is an error to call ErView₋₋ EndRendering() for a particular view object without having first called ErView₋₋ StartRendering() for that view object. However, different view objects can specify the same renderer if desired, since the instance data for the different view objects are separate. The example application program in Appendix A takes a very simple tack in rendering the model twice, specifically by rendering the model completely using the view object previously defined, which specifies the wire frame renderer, then changing the choice of renderer in the view object to point to a Z-buffer renderer, and rendering the model completely once again to the same, now-changed, view object.

Between calls to ErView₋₋ StartRendering() and ErView₋₋ EndRendering(), an application program can make calls to either immediate mode Escher drawing procedures or retained mode Escher drawing procedures, or both. The immediate mode routines take data structures (such as a polygon data structure) as parameters, whereas retained mode routines take objects (such as an EtGeometryObject) as parameters. Immediate mode routines do not instigate a traversal of any model, whereas certain retained mode routines, such as ErDisplayGroup₋₋ Draw(), do instigate a traversal of a model.

A. Traversing a Model

Accordingly, the example application program in Appendix A makes a single call to Escher's renderer invocation procedures 212 (FIG. 2), specifically a call to ErDisplayGroup₋₋ Draw(). The application program passes in the model to render (represented by the ordered display group object group which forms the root node of the model hierarchy) and the view to which the model is to be rendered (by passing the view object view). The application program could at this time also make additional calls to Escher's renderer invocation procedures 212, to render additional objects (including additional models) into the same view. The ErDisplayGroup₋₋ Draw() procedure is as follows:

    __________________________________________________________________________     Copyright © 1994 Apple Computer, Inc.                                  EtStatus EiDisplayGroup.sub.-- Draw(                                           EtDisplayGroupObject                                                                          group,                                                          EtViewObject   viewObject)                                                     StStatus       (*func) (EtDisplayGroupObject, EtViewObject);                   EtViewPrivate  *ViewPrivPriv;                                                  EtDisplayGroupState                                                                           state;                                                          EtStatus       result;                                                         viewPrivPriv = EiView.sub.-- GetView(viewObject);                              EiAssert(viewPrivPriv != NULL);                                                if (viewPrivPriv->cancelRendering)                                              return (EcSuccess);                                                           EiView.sub.-- CheckStarted(viewPrivPriv, "EiDisplayGroup.sub.-- Draw");        EiDisplayGroup.sub.-- GetState(group, &state);                                 if ((state & EcDisplayGroupStateMask.sub.-- IsDrawn) == 0) {                    return (EcSuccess);                                                           }                                                                              func = (Etstatus (*)(EtDisplayGroupObject, EtViewObject))                       EiMethodTable.sub.-- GetMethod(                                                  viewPrivPriv->rendererClass->groupMethods,                                     EiDisplayGroup.sub.-- GetTypeIndex(group));                                 if (func == NULL) {                                                             return (EcSuccess);                                                           }                                                                              if ((state & EcDisplayGroupStateMask.sub.-- IsInline) != 0) {                   return (*func) (group, viewObject);                                           }                                                                              if (EiView.sub.-- PushState(viewObject) != EcSuccess) {                         return (EcFailure);                                                           }                                                                              result = (*func)(group, viewObject);                                           if (EiView.sub.-- PopState(viewObject) != EcSuccess) {                          return (EcFailure);                                                           }                                                                              return result;                                                                 }                                                                              __________________________________________________________________________

Referring to the above procedure, after some error checking, the procedure first determines that group is an ordered display group by passing group to an Escher EiDisplayGroup₋₋ GetTypeIndex() procedure. It then obtains a pointer func to the drawing method which the ordered display group class registered during initialization with the EtDisplayGroup class method tables for ordered display group objects. If the "in-line" flag is set for the object group, then the procedure next calls the procedure pointed to by func, passing in group and the view object to which the group is to be rendered.

If the "in-line" flag for the object group is not set, then the EiDisplayGroup₋₋ Draw() procedure "pushes" the traversal state before calling the procedure identified by func and "pops" the traversal state afterwards.

In one embodiment, the state of traversal is represented as the current position in a stack which may be pushed and popped, and which contains at each level a concatenation of all of the transform matrices prior to that level, the current style characteristics, the current shader characteristics and the current attribute sets. Each time the traversal state is "pushed", a new level is created and all of this information is copied from the prior level to the current level of the stack. Moreover in this embodiment, the concatenation of transform matrices takes place by actually performing each matrix pre-multiplication as the transform object is encountered in the traversal.

In a preferred embodiment, however, the current transform, the current style characteristics, the current attribute sets and the current shader characteristics are stored in a plurality of stacks, each of which is pushed only when necessary. A master "state" stack maintains a record at each level of which of the component stacks need to be popped when the overall traversal state is popped from that level. For example, the current transform state is represented using several component stacks, such as a current "local-to-world" matrix stack, an inverse matrix stack, and so on. But instead of calculating these matrices on each push of the overall traversal state, only the sequence of transformation matrices, from the last-calculated matrix to the current position in traversal of the model, is recorded. The actual calculation is not performed unless and until it is actually needed. In this manner, a large number of matrix computations are avoided.

It should be noted that in retained mode, Escher pushes and pops as required during its traversal of the model. In immediate mode, the application program can also call Escher push and pop routines so that by careful sequencing of calls, the application program can perform its own traversal of its own model database.

Returning to EiDisplayGroup₋₋ Draw (), the procedure pointed to by func is EiView₋₋ OrderedDisplayGroup(), which is as follows:

    __________________________________________________________________________     Copyright © 1994 Apple Computer, Inc.                                  EtStatus EiView.sub.-- OrderedDisplayGroup(                                    EtGroupObject                                                                             group,                                                              EtViewObject                                                                              viewObject)                                                         EtOrderedDisplayGroupPrivate                                                                      *groupdata;                                                 groupData = EiOrderedDisplayGroup.sub.-- GetOrderedDisplayGroup (group);       if (EiDisplayGroupList.sub.-- Draw(groupData->transforms,                                     viewObject,                                                                    EiTransform.sub.-- Draw) == EcFailure                                                              ||                        EiDisplayGroupList.sub.-- Draw(groupData->styles,                                             viewObject,                                                                    EiStyle.sub.-- Draw) == EcFailure                                                                  ||                        EiDisplayGroupList.sub.-- Draw(groupData->attributeSet,                                       viewObject,                                                                    EiAttributeSet.sub.-- Draw) == EcFailure                                                           ||                        EiDisplayGroupList.sub.-- Draw(groupData->shaders,                                            viewObject,                                                                    EiShader.sub.-- Draw) == EcFailure                                                                 ||                        EiDisplayGroupList.sub.-- Draw(groupData->geometries,                                         viewObject,                                                                    EiGeometry.sub.-- Draw) == EcFailure                                                               ||                        EiDisplayGroupList.sub.-- Draw(groupData->groups,                                             viewObject,                                                                    EiDisplayGroup.sub.-- Draw) == EcFailure) {                     return (EcFailure);                                                            }                                                                              return (EcSuccess);                                                            }                                                                              __________________________________________________________________________

As can be seen, the above procedure calls a generalized display group list drawing function EiDisplayGroupList₋₋ Draw() six times, each time passing in a reference to a different one of the six doubly linked lists in the private data of the ordered display group object group. Specifically, the procedure calls the list drawing procedure with a reference first to the list of transforms, then with the reference to the list of styles, then with the reference to the list of attribute sets, then with a reference to the list of shaders, then with a reference to the list of geometries in the group, and then with a pointer to the list of subgroups in the group. Each time, the procedure also passes to the list drawing procedure a reference to the particular procedure which draws the kind of objects which are on the list. For example, when the list of transforms is passed to EiDisplayGroupList₋₋ Draw(), a reference to the Escher procedure EiTransform₋₋ Draw() is also passed. As another example, when the list of geometries is passed to EiDisplayGroupList₋₋ Draw(), a reference to the Escher procedure EiGeometry₋₋ Draw() is also passed in.

It will be appreciated that if group were a list display group object rather than an ordered display group object, only one call would be made to EiDisplayGroupList₋₋ Draw(). This call would pass in a reference to the single list of objects attached to group, and a reference to a procedure which both determines the type of each object as it is encountered on the list, and calls the appropriate Escher drawing procedure for that type.

The generalized list drawing procedure is as follows:

    __________________________________________________________________________     Copyright © 1994 Apple Computer, Inc.                                  EtStatus EiDisplayGroupList.sub.-- Draw(                                       EtDLList   *objectList,                                                        EtViewObject                                                                              viewObject,                                                         EtStatus   (*draw)(                                                                        EtObject object,                                                               EtViewObject                                                                            viewObject))                                              EtGroupPosition                                                                           gPos;                                                               EiAssert(viewObject);                                                          EiAssert(draw);                                                                if ((objectList != NULL) && (EiDLList.sub.-- GetLength (objectList)>0)){         for (gPos =                                                                      (EtGroupPosition)EiDLList.sub.-- GetFirstNode(objectList);                     gPos != NULL;                                                                  gPos = (EtGroupPosition)EiDLListNode.sub.-- Next(objectList,                                   (EtDLListNode *)gPos)) {                                       /* EiView.sub.-- SetCurrentPathIndex(view, a); */                              if ((*draw)(gPos->object, viewObject) != EcSuccess) {                            return (EcFailure);                                                          }                                                                            }                                                                            }                                                                              return (EcSuccess);                                                            }                                                                              __________________________________________________________________________

As can be seen, this procedure merely loops through all of the objects on the doubly linked list which was specified by the caller, and for each such object, passes the object to the drawing procedure specified by the caller.

Several of the Escher object drawing procedures which are called by the list drawing procedure will now be described. However, for convenience of description, they will be described in a different sequence from that in which they would called when rendering an ordered display group.

B. Drawing Subgroup Objects of a Display Group Object

Escher traverses the model in a recursive manners and for this reason, the Escher procedure which ErView₋₋ OrderedDisplayGroup() passes to the list drawing procedure for group objects encountered in the group object group, is simply EiDisplayGroup₋₋ Draw(). This procedure is described above.

C. Drawing Geometry Objects in a Display Group Object

The Escher procedure which EiView₋₋ OrderedDisplayGroup() identifies to the list drawing routine for geometry objects is EiGeometry₋₋ Draw(), which is as follows:

    __________________________________________________________________________     Copyright © 1994 Apple Computer, Inc.                                  EtStatus EiGeometry.sub.-- Draw(                                               EtGeometryObject                                                                            geometry,                                                         EtViewObject viewObject)                                                       EtViewPrivate  *view;                                                          view = EiView.sub.-- GetView(viewObject);                                      EiAssert(view != NULL);                                                        if (view->cancelRendering) {                                                   return (EcSuccess);                                                            }                                                                              EiView.sub.-- CheckStarted(view, "EiGeometry.sub.-- Draw");                    return ((*view->rendererClass->geometryDrawMethod)(geometry,                                             viewObject));                                        }                                                                              __________________________________________________________________________

As can be seen, the above procedure merely performs certain error checking and then calls the geometry draw method which was registered by the geometry class. This method is generic to all geometry objects, and is as follows:

    __________________________________________________________________________     Copyright © 1994 Apple Computer, Inc.                                  static EtStatus EiGeometry.sub.-- DrawMethod(                                  EtGeometryObject                                                                            geometry,                                                         EtViewObject viewObject)                                                       void       (*func) (                                                                        EtGeometryObject                                                                          geometry,                                                           EtViewObject                                                                              view);                                                 EtViewPrivate  *view;                                                          view = EiView.sub.-- GetView(viewObject);                                      EiView.sub.-- CheckStarted(view, "EiGeometry.sub.-- Draw");                    func =                                                                         EiMethodTable.sub.-- CheckMethod(view->rendererClass->geometryDrawMethods,     3                                                                              EiGeometry.sub.-- GetTypeIndex(geometry));                                     if (func) {                                                                    (*func) (geometry, viewObject);                                                } else {                                                                       EiGeometry.sub.-- Decompose(geometry, viewObject);                             }                                                                              return (EcSuccess);                                                            }                                                                              __________________________________________________________________________

This routine, after error checking, first obtains a pointer func to the method which the current view's renderer has registered for geometry objects of the type to be rendered (in this case, polygons). If func is NULL, then a decomposition of the geometry object takes place in a manner hereinafter described. In the present case, however, the wire frame renderer has registered ErWF₋₋ Geometry₋₋ Polygon() as the procedure to render polygon objects. (See the discussion above with respect to ErWF₋₋ Register().) That procedure is set out in Appendix B.

D. Drawing Geometry Objects Which Require Decomposition

In the previous section, it was described how Escher causes a geometry object in a display group to be rendered in the situation where the view's renderer has a routine specifically for the geometry object's type (i.e., polygon). Renderers to be used with Escher must at least support routines which will render points, lines and triangles, and may also support routines which will render higher level geometries. In an aspect of the invention, the renderer need not support all geometry types which Escher supports in the building of a model. Rather, Escher will automatically detect the absence of a rendering method for a particular geometry type, and decompose the geometry into less complex parts. It then resubmits them to the renderer in this less complex form.

The wire frame renderer of the present embodiment supports polygon objects, but for purposes of illustration, it will now be assumed that it does not. This illustration is hypothetical also because the wire frame renderer's method for rendering triangles, which as will be seen is the geometry to which Escher decomposes polygons, is the same wire frame renderer procedure which renders polygons. That is, the renderer procedure ErWF₋₋ Geometry₋₋ Polygon() takes either a triangle or a polygon as an argument, determines which it is, and renders it using the same code. Nevertheless, the hypothetical will serve to illustrate Escher's decomposition technique.

1. Procedures Called On Escher Initialization

The procedure upon initialization for registering a renderer has been previously described. Classes and subclasses register themselves as well during initialization. The polygon class, for example, is registered initially by calling the following function:

    __________________________________________________________________________     Copyright © 1994 Apple Computer, Inc.                                  EtStatus EiPolygon.sub.-- Register(                                            void)                                                                          EtObjectClassData  objectClassData;                                            EgPolygonClass =                                                                 EiGeometryClass.sub.-- Register(                                                 EiObjectClassData.sub.-- Initialize(                                             &objectClassData,                                                              EcGeometryType.sub.-- Polygon, "Polygon",                                      EiPolygon.sub.-- MetaHandler, NULL,                                            0));                                                                     return ((EgPolygonClass == NULL) ? EcFailure : EcSuccess);                     }                                                                              __________________________________________________________________________

This routine creates the class method tables for an object in the polygon leaf class. As with other creation routines previously described, the block of memory is allocated and initialized using a layered technique, with each class's class registration procedure first passing the size required for its own method tables to its parent class's class registration procedure. The polygon class is a leaf class, so the size of its method table is zero. Above the polygon class, each procedure adds on the size of its own method table and recursively calls its parent class's class registration procedure. The ultimate parent class's class registration procedure allocates memory for the entire block of method tables, initializes its own portion with pointers to its default methods and returns to the calling subclass's class registration procedure.

On the way back down to the original callers each class's class registration procedure initializes its own method table with its own default methods. In addition, each class's class registration procedure can specify methods to override those of its parent class, but only at the option of the parent class. This is accomplished by, when calling the parent class's class registration procedure, specify a metahandler and, for some classes, a virtual metahandler, as arguments to the call. These metahandlers can be called by the parent class's class registration procedure, if it so desires, specifying a method type, and if called, the metahandler returns a pointer to the procedure desired to override the parent class's default method of the type specified. If a class does not have a procedure to override the parent class's default method of a particular type, the metahandler returns NULL.

In the case of the polygon class registration procedure, only a metahandler is identified to the parent class's class registration procedure. The metahandler is as follows:

    __________________________________________________________________________     Copyright © 1994 Apple Computer, Inc.                                  static EtFunctionPointer EiPolygon.sub.-- MetaHandler(                         EtMethodType   methodType)                                                     switch (methodType) {                                                          case EcMethodType.sub.-- ObjectDelete:                                         return (EtFunctionPointer) EiPolygon.sub.-- Delete;                            case EcMethodType.sub.-- ObjectDuplicate:                                      return (EtFunctionPointer) EiPolygon.sub.-- Duplicate;                         case EcMethodType.sub.-- BoundingBox:                                          return (EtFunctionPointer) EiPolygon.sub.-- BoundingBox;                       case EcMethodType.sub.-- BoundingSphere:                                       return (EtFunctionPointer) EiPolygon.sub.-- BoundingSphere;                    case EcMethodType.sub.-- GeometryRayPick:                                      return (EtFunctionPointer) EiPolygon.sub.-- RayPick;                           case EcMethodType.sub.-- GeometryBoxPick:                                      return (EtFunctionPointer) EiPolygon.sub.-- BoxPick;                           case EcMethodType.sub.-- GeometrySpherePick:                                   return (EtFunctionPointer) EiPolygon.sub.-- SpherePick;                        case EcMethodType.sub.-- GeometryWindowPointPick:                              return (EtFunctionPointer) EiPolygon.sub.-- WindowPointPick;                   case EcMethodType.sub.-- GeometryWindowRectPick:                               return (EtFunctionPointer) EiPolygon.sub.-- WindowRectPick;                    case EcMethodType.sub.-- Decompose:                                            return (EtFunctionPointer) EiPolygon.sub.-- Decompose;                         case EcMethodType.sub.-- DeleteDecomposition:                                  return (EtFunctionPointer) EiPolygon.sub.-- DeleteDecomposition;               case EcMethodType.sub.-- GetAttributeSet:                                      return (EtFunctionPointer) EiPolygon.sub.-- GetAttributeSet;                   case EcMethodType.sub.-- SetAttributeSet:                                      return (EtFunctionPointer) EiPolygon.sub.-- SetAttributeSet;                   case EcMethodType.sub.-- ObjectRead:                                           return (EtFunctionPointer) EiPolygon.sub.-- Read;                              case EcMethodType.sub.-- ObjectWrite:                                          return (EtFunctionPointer) EiPolygon.sub.-- Write.sub.-- Internal;             case EcMethodType.sub.-- ObjectTraverse:                                       return (EtFunctionPointer) EiPolygon.sub.-- Traverse;                          default:                                                                       return (EtFunctionPointer) NULL;                                               }                                                                              }                                                                              __________________________________________________________________________

Of particular relevance to the present discussion, note that when asked for a geometry decomposition procedure, the metahandler returns one, namely EiPolygon₋₋ Decompose. This procedure is described hereinafter.

The class registration procedure for the polygon class's parent class, EtGeometryObject, is as follows:

    __________________________________________________________________________     Copyright © 1994 Apple Computer, Inc.                                  EtObjectClass EiGeometryClass.sub.-- Register(                                 EtObjectClassData *objectClassData)                                            EtObjectClass                                                                               objectClass;                                                      EtGeometryClass                                                                             *geomClass;                                                       EtObjectClassData                                                                           parentObjectClassData;                                            objectClass =                                                                  EiShapeClass.sub.-- Register(                                                  EiObjectClassData.sub.-- Add(                                                  EiObjectClassData.sub.-- ParentInitialize(                                     &parentObjectClassData,                                                        EcShapeType.sub.-- Geometry,                                                   "Geometry",                                                                    EiGeometryClass.sub.-- MetaHandler,                                            EiGeometryClass.sub.-- VirtualMetaHandler, NULL,                               sizeof(EtGeometryClass)),                                                      objectClassData));                                                             if (objectClass == NULL) {                                                     return (NULL);                                                                 }                                                                              geomClass = EiShapeClass.sub.-- GetSubClassMethods(objectClass,                                       EcShapeType.sub.-- Geometry);                           /*                                                                              * EgGeometryUniqueNumber is used in method tables in view                      */                                                                            geomClass->geometryUniqueNumber = EgNumGeometryClasses;                        geomClass->boundingBox = (EtBoundingBoxMethod)                                 EiObjectClassData.sub.-- GetMethod(objectClassData,                                             EcMethodType.sub.-- BoundingBox);                             geomClass->boundingSphere = (EtBoundingSphereMethod)                           EiObjectClassData.sub.-- GetMethod(objectClassData,                                             EcMethodType.sub.-- BoundingSphere);                          geomClass->rayPick = (EtGeometryPickMethod)                                    EiObjectClassData.sub.-- GetMethod(objectClassData,                                             EcMethodType.sub.-- GeometryRayPick);                         geomClass->boxPick = (EtGeometryPickMethod)                                    EiObjectClassData.sub.-- GetMethod(objectClassData,                                             EcMethodType.sub.-- GeometryBoxPick);                         geomClass->spherePick = (EtGeometryPickMethod)                                 EiObjectClassData.sub.-- GetMethod(objectClassData,                                             EcMethodType.sub.-- GeometrySpherePick);                      geomClass->windowPointPick = (EtGeometryPickMethod)                            EiObjectClassData.sub.-- GetMethod(objectClassData,                                             EcMethodType.sub.-- GeometryWindowPointPick);                 geomClass->windowRectPick = (EtGeometryPickMethod)                             EiObjectClassData.sub.-- GetMethod(objectClassData,                                             EcMethodType.sub.-- GeometryWindowRectPick);                  geomClass->decompose = (EtDecomposeMethod)                                     EiObjectClassData.sub.-- GetMethod(objectClassData,                                             EcMethodType.sub.-- Decompose);                               geomClass->deleteDecomposition = (EtObjectDisposeMethod)                       EiObjectClassData.sub.-- GetMethod(objectClassData,                                             EcMethodType.sub.-- DeleteDecomposition);                     geomClass->getAttributeSet = (EtGetAttributeSetMethod)                         EiObjectClassData.sub.-- GetMethod(objectClassData,                                             EcMethodType.sub.-- GetAttributeSet);                         geomClass->setAttributeSet = (EtSetAttributeSetMethod)                         EiObjectClassData.sub.-- GetMethod(objectClassData,                                             EcMethodType.sub.-- SetAttributeSet);                         if (geomClass->decompose == NULL) {                                            #ifdef VERBOSE                                                                 EiWarning.sub.-- Post(EcError.sub.-- NoRecovery,                               EiWarning.sub.-- Message(                                                      "%s: Geometry registered a NULL decompose function",                           "EiGeometry.sub.-- Register"));                                                #endif                                                                         }                                                                              if (geomClass->deleteDecomposition == NULL) {                                  #ifdef VERBOSE                                                                 EiWarning.sub.-- Post(EcError.sub.-- NoRecovery,                                          EiWarninig.sub.-- Message(                                          "%s: Geometry registered a NULL deleteDecomposition function",                             "EiGeometry.sub.-- Register"));                                    #endif                                                                         }                                                                              if (geomClass->rayPick == NULL) {                                              #ifdef VERBOSE                                                                 EiWarning.sub.-- Post(EcError.sub.-- NoRecovery,                               EiWarning.sub.-- Message(                                                      "%s: Geometry registered a NULL ray pick function",                            "EiGeometry.sub.-- Register"));                                                #endif                                                                         geomClass->rayPick = EiGeometry.sub.-- DefaultPick;                            }                                                                              if (geomClass->boxPick == NULL) {                                              #ifdef VERBOSE                                                                 EiWarning.sub.-- Post(EcError.sub.-- NoRecovery,                               EiWarning.sub.-- Message(                                                      "%s: Geometry registered a NULL box pick function",                            "EiGeometry.sub.-- Register"));                                                #endif                                                                         geomClass->boxPick = EiGeometry.sub.-- DefaultPick;                            }                                                                              if (geomClass->spherePick == NULL) {                                           ifdef VERBOSE                                                                  EiWarning.sub.-- Post(EcError NoRecovery,                                      EiWarning.sub.-- Messaage(                                                     "%s: Geometry registered a NULL sphere pick function",                         "EiGeometry.sub.-- Register"));                                                #endif                                                                         geomClass->spherePick = EiGeometry.sub.-- DefaultPick;                         }                                                                              if (geomClass->windowPointPick == NULL) {                                      ifdef VERBOSE                                                                  EiWarning.sub.-- Post(EcError.sub.-- NoRecovery,                               EiWarning.sub.-- Message("%s: Geometry registered a NULL                       window point pick function",                                                   "EiGeometry.sub.-- Register"));                                                #endif                                                                         geomClass->windowPointPick = EiGeometry.sub.-- DefaultPick;                    }                                                                              if (geomClass->windowRectPick == NULL) {                                       #ifdef VERBOSE                                                                 EiWarning.sub.-- Post(EcError.sub.-- NoRecovery,                               EiWarning.sub.-- Message("%s: Geometry registered a NULL                       window rect pick function",                                                    "EiGeometry.sub.-- Register"));                                                #endif                                                                         geomClass->windowRectPick = EiGeometry.sub.-- DefaultPick;                     }                                                                              /*                                                                              * Not legal to register NULL bbox method                                       */                                                                            if (geomClass->boundingBox == NULL) {                                          EiError.sub.-- Post(EcError.sub.-- NoRecovery,                                 EiError.sub.-- Message("%s: Geometry registered a NULL                         bounding box function",                                                        "EiGeometry.sub.-- Register"));                                                geomClass->boundingBox = EiGeometry.sub.-- DefaultBoundingBox;                 }                                                                              if (geomClass->boundingSphere == NULL) {                                       #ifdef VERBOSE                                                                 EiWarning.sub.-- Post(EcError.sub.-- NoRecovery,                               EiWarning.sub.-- Message("%s: Geometry registered a NULL                       bounding sphere function",                                                     "EiGeometry.sub.-- Register"));                                                #endif                                                                         geomClass->boundingSphere = EiGeometry.sub.-- DefaultBoundingSphere;           }                                                                              EgGeometryClasses =                                                            EiMemory.sub.-- Realloc(                                                       EgGeometryClasses,                                                             (EgNumGeometryClasses + 1) * sizeof(EtGeometryClass *));                       if (EgGeometryClasses == NULL) {                                               EiShapeClass.sub.-- Unregister(objectClass);                                   return (NULL);                                                                 }                                                                              EgGeometryClasses[EgNumGeometryClasses++] = objectClass;                       if (EiObjectClass.sub.-- FileMethodsExist(objectClass) == EcTrue)              {                                                                              if (EiObjectClass.sub.-- RegisterAttachmentMethod(                             objectClass,                                                                   EcSetType.sub.-- Attribute,                                                    EiGeometry.sub.-- AttachAttributeSet) == EcFailure)                            {                                                                              Eiwarning.sub.-- Post(EcError.sub.-- InternalError,                            "Can't register geometry attachment");                                         }                                                                              }                                                                              return (objectClass);                                                          }                                                                              __________________________________________________________________________

As can be seen, this procedure calls its own parent class's class registration procedure, EiShapeClass₋₋ Register(), specifying a metahandler and a virtual metahandler. After the recursion returns to the above procedure, the procedure continues by initializing its own method table with pointers to default procedures. It obtains these pointers using an Escher procedure, EiObjectClassData₋₋ GetMethod(), which among other things, asks the subclass's metahandler to provide the pointer to each desired method. One of the methods which the procedure asks the polygon class's metahandler to identify is a decomposition method, and as previously pointed out, the method which it identifies is EiPolygon₋₋ Decompose().

It is noteworthy that in Escher, the decomposition method is identified in the class method tables for each type of geometry, not for each renderer. Thus the decomposition methods are included as part of the model to be rendered, not as part of the view to which it will be rendered or as part of the renderer. It will be appreciated that in another embodiment, a renderer can be permitted to override the decomposition methods for geometry which it cannot draw directly. In yet another embodiment, the decomposition methods can be attached to the renderer subclass rather than to the geometry subclass.

A description of the geometry classes metahandler is not important for an understanding of the invention, but the geometry classes virtual metahandler is set out below:

    __________________________________________________________________________     Copyright © 1994 Apple Computer, Inc.                                  EtFunctionPointer EiGeometryClass.sub.-- VirtualMetaHandler(                   EtMethodType methodType)                                                       switch (methodType) {                                                          case EcMethodType.sub.-- ObjectUnregister:                                     return ((EtFunctionPointer)EiGeometryClass.sub.-- Unregister);                 case EcMethodType.sub.-- ObjectDraw:                                           return ((EtFunctionPointer)EiGeometry.sub.-- Draw);                            default:                                                                       return (NULL);                                                                 }                                                                              }                                                                              __________________________________________________________________________

Of particular relevance is that when called by the EtShapeObject class's class registration procedure, it will return a pointer to EiGeometry₋₋ Draw() as an object drawing procedure.

2. Procedures Called During Rendering

Returning to EiGeometry₋₋ DrawMethod() set out above, as previously mentioned, if the renderer has not registered a procedure for drawing geometries of the type provided to that procedure, which for the purposes of illustration we are assuming is the case with respect to polygons, then this routine will pass the geometry on to EiGeometry₋₋ Decompose(). That procedure is as follows:

    ______________________________________                                         Copyright © 1994 Apple Computer, Inc.                                  EtStatus EiGeometry.sub.-- Decompose(                                          EtGeometryObject                                                                              geometry,                                                       EtViewObject   view)                                                           EtGeometryClass                                                                               *theClass;                                                      theClass = (EtGeometryClass *)                                                 EiShape.sub.-- GetSubClassMethods(geometry, EcShapeType.sub.-- Geometry);      1                                                                              EiAssert(theClass);                                                            if (!theClass->decompose) {                                                    /*                                                                              * Some geoms can't be decomposed (e.g., lines, triangles).                     * If this code path is hit, that means that somebody is                        * trying to decompose a primitive that ought not be                            * decomposed (or cannot). Unless a picking or bounding box                     * routine is erroneously trying to decompose a point, line,                    * triangle, or marker, then it must be that a renderer is                      * not providing a pipeline for the geometric primitive in                      * question. This is an error . . .                                             */                                                                            EiError.sub.-- Post(                                                           EcError.sub.-- Unimplemented,                                                  EiError.sub.-- Message(                                                        "%s: Renderer does not support a required geometric                            primitive.",                                                                   "EiGeometry.sub.-- Decompose"));                                               return (EcSuccess);                                                            }                                                                              EiAssert(theClass->decompose);                                                 return ((*theClass->decompose) (geometry, view));                              }                                                                              ______________________________________                                    

As can be seen, after some error checking, this procedure merely passes the geometry object on to the method identified in its method table for a decomposition procedure. For polygons, this method was previously registered as described above as EiPolygon₋₋ Decompose, which is as follows:

    __________________________________________________________________________     Copyright © 1994 Apple Computer, Inc.                                  static EtStatus EiPolygon.sub.-- Decompose(                                    EtGeometryObject                                                                            polygon,                                                          EtViewObject view)                                                             EtPolygonPrivate                                                                            *polygonPriv;                                                     EtBoolean    immediateMode = EcFalse;                                          if (!(polygonPriv = EiPolygon.sub.-- GetPolygon(polygon))) {                   return (EcFailure);                                                            }                                                                              if (immediateMode) {                                                           if ((EiPolygon.sub.-- Triangulate(polygon, EcFalse, NULL, view)) ==                                     EcFailure) {                                          return (EcFailure);                                                            }                                                                              } else {                                                                       if (!polygonPriv->decomposition) {                                             if ((EiPolygon.sub.-- Triangulate(polygon,                                                     EcTrue,                                                                        &polygonPriv->decomposition,                                                   view)) == EcFailure) {                                         return (EcFailure);                                                            }                                                                              }                                                                              if ((EiGeometry.sub.-- Draw(polygonPriv->decomposition, view)) ==                                       EcFailure) {                                          return (EcFailure);                                                            }                                                                              }                                                                              return (EcSuccess);                                                            }                                                                              __________________________________________________________________________

The above procedure uses an Escher procedure EiPolygon₋₋ Triangulate() to decompose the polygon into a bundle of triangles. In one embodiment, the triangulation procedure could create a new EtGroupObject to hold all of the new triangles. Preferably, however, in order to shortcircuit a level of error checking and recursion, since it is already known that all objects within the group will be geometry objects, the triangulation procedure places all of the new triangles in an object of class EtGeometryBundle. A geometry bundle is an internal-only geometry object which can hold only a list of geometries.

The above procedure can operate either in immediate mode or in retained mode. In immediate mode, it calls the triangulation procedure with a flag that tells the triangulation procedure not to save the result of the decomposition, but rather to simply have it drawn. In retained mode, the above procedure calls the triangulation procedure with a flag indicating that the decomposition should be saved, as well as with a reference to a field in which to save it. In the above procedure, the field into which the triangulation procedure writes a reference to the resulting geometry bundle object, is polyPriv->decomposition, which the above procedure then passes to EiGeometry₋₋ Draw().

EiGeometry₋₋ Draw() has been set out above, and in the manner previously described, after error checking, it merely calls the method identified in the method table of the geometry object passed to it, for drawing that type of geometry. Usually these will be methods registered by the renderer, but geometry bundle objects are not public. Renderers do not register routines to draw these objects. Instead, the method table will contain a pointer to Escher's default drawing method for geometry bundles, which is as follows:

    ______________________________________                                         Copyright © 1994 Apple Computer, Inc.                                  void EiView.sub.-- GeometryBundle(                                             EtGeometryObject                                                                              geometry,                                                       EtViewObject   view)                                                           EtGeometryBundlePrivate                                                                          *bundle;                                                     long              i;                                                           if (!(bundle = (EtGeometryBundlePrivate *)                                            EiGeometry.sub.-- GetSubClassData(geometry,                                    EcGeometryType.sub.-- GeometryBundle))) {                               return;                                                                        }                                                                              for (i = 0; i < bundle->numGeometries; i++) {                                  EiGeometry.sub.-- Draw(bundle->geometries[i], view);                           }                                                                              }                                                                              ______________________________________                                    

As can be seen, this procedure merely iterates through the list of geometry objects in the geometry bundle object provided to it, and passes each one in turn to EiGeometry₋₋ Draw(). These are all triangles, so EiGeometry₋₋ Draw() will call the renderer's triangle drawing procedure for each triangle in the decomposition.

E. Drawing Transform Objects in a Display Group Object

The Escher procedure which EiView₋₋ OrderedDisplayGroup() identifies to the list drawing routine for transform objects is EiTransform₋₋ Draw(), which is as follows:

    ______________________________________                                         Copyright © 1994 Apple Computer, Inc.                                  EtStatus EiTransform Draw(                                                     EtTransformObject                                                                             t,                                                              EtViewObject   viewObject)                                                     EtViewPrivate  *view;                                                          view = EiView.sub.-- GetView(viewObject);                                      return ((*view->rendererClass->transformDrawMethod)                                                  (t, viewObject));                                        }                                                                              ______________________________________                                    

As can be seen, the above procedure merely calls the transform draw method for the EtTransformObject class. This method is generic to all transform objects, and is as follows:

    __________________________________________________________________________     Copyright © 1994 Apple Computer, Inc.                                  static EtStatus EiTransform.sub.-- DrawMethod(                                 EtTransformObject                                                                           t,                                                                EtViewObject viewObject)                                                       void (*func) (                                                                 EtTransformObject                                                                             transform,                                                      EtViewObject   viewObject);                                                    EtViewPrivate  *view;                                                          view = EiView.sub.-- GetView(viewObject);                                      EiView.sub.-- CheckStarted(view, "EiView.sub.-- Transform");                   func = EiMethodTable.sub.-- GetMethod                                                     (view->rendererClass->transformDrawMethods,                                        EiTransform.sub.-- GetTypeIndex(t));                            if (func) {                                                                    (*func)(t, viewObject);                                                        }                                                                              return (EcSuccess);                                                            }                                                                              __________________________________________________________________________

This routine, after error checking, first obtains a pointer func to the method which the current view's renderer's method table has for executing transform objects of the type to be executed (in this case, translations). Escher's default procedure for executing translation transforms is as follows:

    __________________________________________________________________________     Copyright © 1994 Apple Computer, Inc.                                  void EiView.sub.-- Translate(                                                  EtTransformObject                                                                           trans,                                                            EtViewObject viewObject)                                                       EtTranslatePrivate                                                                           *t;                                                              EtViewTransformState                                                                         *newState, *state;                                               EtViewPrivate *view;                                                           view = EiView.sub.-- GetView(viewObject);                                      if (!(t = (EtTranslatePrivate *)                                               EiTransform.sub.-- GetSubClassData(trans,                                                         EcTransformType.sub.-- Translate))) {                       return;                                                                        }                                                                              if (EiAttributeStack.sub.-- Check(                                             view->renderer->transforms->stackTransformation)) {                            newState = EiMemory.sub.-- New(sizeof(EtViewTransformState));                  if (!newState) {                                                               return;                                                                        }                                                                              /* indicates that inverse etc has not been calculated */                       newState->flags = 0;                                                           /*   We want new = translate * current  */                                     state = (EtViewTransfomState *) EiAttributeStack.sub.-- GetTop(                view->renderer->transforms->stackTransformation);                              EiMatrix4×4.sub.-- Copy(&state->localToWorldMatrix,                                         &newState->localToWorldMatrix);                             {                                                                              EtMatrix4×4                                                                               tmp;                                                          EiMatrix4×4.sub.-- SetTranslate(&tmp,                                                       t->translate.x,                                                                t->translate.y,                                                                t->translate.z);                                            EiMatrix4×4.sub.-- Multiply(&tmp,                                                         &newState->localToWorldMatrix,                                                 &newState->localToWorldMatrix);                               }                                                                              EiAttributeStack.sub.-- Push(                                                  view->renderer->transforms->stackTransformation,                                               (void *)newState,                                                              EiView.sub.-- Transformation.sub.-- Pop);                      } else {                                                                       /* We want current = translate * current  */                                   state = (EtViewTransformState *) EiAttributeStack.sub.-- GetTop(               view->renderer->transforms->stackTransformation);                              {                                                                              EtMatrix4×4                                                                               tmp;                                                          EiMatrix4×4.sub.-- SetTranslate(&tmp,                                                       t->translate.x,                                                                t->translate.y,                                                                t->translate.z);                                            EiMatrix4×4.sub.-- Multiply(&tmp,                                                         &state->localToWorldMatrix,                                                    &state->localToWorldMatrix);                                  }                                                                              /* indicates that inverse etc has not been calculated */                       state->flags = 0;                                                              }                                                                              /* notify renderers that current trnsfrmatn matrix has changed */              EiView.sub.-- Transform.sub.-- StateChanged(                                   viewObject,                                                                    EcViewStateType.sub.-- Transform.sub.-- LocalToWorldMatrix);                   }                                                                              __________________________________________________________________________

The foregoing description of preferred embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents. 

I claim:
 1. A method for creating a representation of a scene displayable on a graphical output device, comprising the steps of:providing in memory a representation of a first object; invoking an object-drawing subsystem with an identification of said first object, said object drawing subsystem performing at least part of a rendering of said first object into said scene and returning a flag indicating whether said rendering of said first object is complete; and repeating said step of invoking if said flag indicates that said rendering of said first object is not complete.
 2. A method according to claim 1, wherein said first object comprises a model containing a plurality of sub-objects.
 3. A method according to claim 1, further comprising, after said step of invoking, the step of further invoking said object-drawing subsystem with an identification of a second object, said object drawing subsystem performing at least part of a rendering of said second object into said scene.
 4. A method according to claim 3, wherein said object drawing subsystem returns said flag only after both of said steps of invoking.
 5. A method according to claim 1, wherein said step of invoking comprises the steps of:first calling said object-drawing subsystem with said identification of said first object, said object drawing subsystem performing said at least part of a rendering of said first object into said scene in response to said first call; and subsequently calling an end-rendering procedure of said object-drawing subsystem, said object drawing subsystem returning said flag in response to said subsequent call.
 6. A method according to claim 5, further comprising, after said step of first calling and before said step of subsequently calling, the step of second calling said object drawing subsystem with an identification of a second object, said object drawing subsystem performing at least part of a rendering of said second object into said scene in response to said second call.
 7. A method according to claim 1, wherein said object drawing subsystem returns said flag indicating that said rendering of said first object is not complete.
 8. A method according to claim 1, wherein said step of invoking an object-drawing subsystem comprises the step of invoking said object-drawing subsystem further with an identification of a first renderer to use to draw said first objects and wherein said object drawing subsystem uses said first renderer to perform said at least part of a rendering.
 9. A method according to claim is further comprising the step of outputting said scene to said graphical output device.
 10. A method according to claim 1, wherein said step of repeating comprises the step of repeating said step of invoking until said flag indicates that said rendering of said first object is complete.
 11. A method for creating a scene displayable on a graphical output device, comprising the steps of, in sequence:starting execution of an application program; establishing for a first renderer, an indication of at least one respective method of said first renderer to be invoked to render objects having a respective geometry; invoking, for an object having a first geometry which said first renderer is able to render, the method of said first renderer established in said step of establishing for rendering objects having said first geometry, said first renderer returning a flag indicating whether rendering of said first object is complete; and repeating said step of invoking if said flag indicates that said rendering of said first object is not complete.
 12. Apparatus for creating a representation of a scene displayable on a graphical output device, for use with and object drawing subsystem and with a memory containing a representation of a first object, comprising:first invoking means for invoking said object-drawing subsystem with an identification of said first object, said object drawing subsystem performing at least a first part of a rendering of said first object into said scene and returning a flag indicating whether said rendering of said first object is complete; and second invoking means for invoking said object-drawing subsystem a second time with an identification of said first object, if said flag indicates that said rendering of said first object is not complete, said object drawing subsystem performing at least a second part of said rendering of said first object into said scene in response to being invoked said second time.
 13. Apparatus according to claim 12, wherein said first invoking means comprises:first means for calling said object-drawing subsystem with said identification of said first object, said object drawing subsystem performing said first part of a rendering of said first object into said scene in response to said first call; and means for subsequently calling an end-rendering procedure of said object-drawing subsystem, said object drawing subsystem returning said flag in response to said subsequent call.
 14. Apparatus according to claim 12, wherein said first invoking means comprises means for identifying to said object-drawing subsystem a first renderer to use to draw said first object, and wherein said object drawing subsystem uses said first renderer to perform said first part of a rendering.
 15. A method according to claim 12, further comprising means for outputting said scene to said graphical output device.
 16. A method according to claim 12, wherein said first and second invoking means collectively comprise means for invoking said object-drawing subsystem repeatedly with an identification of said first object, until said flag indicates that said rendering of said first object is complete. 